tri: restore cost and diffbin, and the same 634 specs now report 13 field losses where the old tool reported zero

.github/workflows/loop-tools-gate.yml

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+# A loop tool that exists only in one working directory is not in the project.
+#
+# scripts/tri_loop/cost.py and diffbin.py were written, used to produce numbers
+# that were quoted in a pull request, and lost -- never committed, and the working
+# copy was later re-cloned. Every figure they had produced became unreproducible
+# at once. Six recovery routes came back empty (#2158).
+#
+# Nothing in CI could tell the difference between a tool that exists and a tool
+# that exists nowhere but one untracked directory. This gate is that difference.
+# It is deliberately cheap: no build, no compiler, no corpus run, so there is no
+# reason for it to be skipped or made optional.
+name: loop-tools-gate
+
+on:
+  pull_request:
+    paths:
+      - "scripts/tri"
+      - "scripts/tri_loop/**"
+      - "scripts/ci/loop-tools-tracked.sh"
+      - ".github/workflows/loop-tools-gate.yml"
+  push:
+    branches: [master]
+    paths:
+      - "scripts/tri"
+      - "scripts/tri_loop/**"
+      - "scripts/ci/loop-tools-tracked.sh"
+
+permissions:
+  contents: read
+
+jobs:
+  loop-tools-tracked:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # The check reads git index state, so it needs real history rather than
+          # a detached blob export.
+          fetch-depth: 1
+
+      - name: Every loop tool is present, tracked, and routed
+        run: bash scripts/ci/loop-tools-tracked.sh
+
+      - name: Each loop helper is syntactically loadable
+        # A tracked tool that cannot be imported is tracked and useless. This is
+        # a syntax check only and asserts nothing about behaviour.
+        run: |
+          for f in scripts/tri_loop/*.py; do
+            python3 -c "import py_compile,sys; py_compile.compile('$f', doraise=True)" \
+              && echo "ok       $f" \
+              || { echo "BROKEN   $f"; exit 1; }
+          done
+
+      - name: The dispatcher runs its helpers without a built compiler
+        # tri triage and tri damage read the tracker and the spec text and have no
+        # use for t27c. The dispatcher used to look for the binary first and
+        # refuse to run them on a machine with no build, so this step exists to
+        # keep that ordering from coming back. There is no compiler in this job,
+        # which is the whole point.
+        run: |
+          ./scripts/tri loop-help
+          ./scripts/tri damage specs >/dev/null || true
+          echo "ok       helpers dispatch with no t27c present"

bootstrap/tests/fixtures/damage/damage_class_01.t27

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+module damage_class_01
+
+// shape: [[]X",
+// 52 line(s) in the corpus share this shape
+// first seen: specs/tri/pipeline/builder.t27:14
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    items : [[]T",
+}

bootstrap/tests/fixtures/damage/damage_class_02.t27

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+module damage_class_02
+
+// shape: [[]X X",
+// 37 line(s) in the corpus share this shape
+// first seen: specs/tri/pipeline/spec_writer.t27:14
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    field_type : [[]Const u8",
+}

bootstrap/tests/fixtures/damage/damage_class_03.t27

@@ -0,0 +1,9 @@
+module damage_class_03
+
+// shape: [[]X [,
+// 10 line(s) in the corpus share this shape
+// first seen: specs/tri/pipeline/spec_writer.t27:23
+// signals: doubled-bracket
+pub struct Damaged {
+    steps : [[]Const [,
+}

bootstrap/tests/fixtures/damage/damage_class_04.t27

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+module damage_class_04
+
+// shape: [[][,
+// 7 line(s) in the corpus share this shape
+// first seen: specs/tri/encoding/mime.t27:15
+// signals: doubled-bracket
+pub struct Damaged {
+    to : [[][,
+}

bootstrap/tests/fixtures/damage/damage_class_05.t27

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+module damage_class_05
+
+// shape: [[9]X",
+// 5 line(s) in the corpus share this shape
+// first seen: specs/tri/crypto/hmac.t27:14
+// signals: odd-quote
+pub struct Damaged {
+    opad : [[64]U8",
+}

bootstrap/tests/fixtures/damage/damage_class_06.t27

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+module damage_class_06
+
+// shape: [?[]X X",
+// 3 line(s) in the corpus share this shape
+// first seen: specs/tri/pipeline/spec_writer.t27:36
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    error_msg : [?[]Const u8",
+}

bootstrap/tests/fixtures/damage/damage_class_07.t27

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+module damage_class_07
+
+// shape: [X.X([]X X)",
+// 2 line(s) in the corpus share this shape
+// first seen: specs/tri/encoding/html.t27:15
+// signals: odd-quote
+pub struct Damaged {
+    attributes : [std.StringHashMap([]Const u8)",
+}

bootstrap/tests/fixtures/damage/damage_class_08.t27

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+module damage_class_08
+
+// shape: [[9]?X",
+// 2 line(s) in the corpus share this shape
+// first seen: specs/tri/trees/octree.t27:24
+// signals: odd-quote
+pub struct Damaged {
+    children : [[8]?OctNode",
+}

bootstrap/tests/fixtures/damage/damage_class_09.t27

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+module damage_class_09
+
+// shape: [[]?X",
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/graph/dijkstra.t27:15
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    parent : [[]?Usize",
+}

bootstrap/tests/fixtures/damage/damage_class_10.t27

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+module damage_class_10
+
+// shape: [X.X(X, []X)",
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/graph/graph.t27:14
+// signals: odd-quote
+pub struct Damaged {
+    nodes : [std.HashMap(T, []T)",
+}

bootstrap/tests/fixtures/damage/damage_class_11.t27

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+module damage_class_11
+
+// shape: [[]?*X",
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/trees/b_tree.t27:15
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    children : [[]?*BTreeNode",
+}

bootstrap/tests/fixtures/damage/damage_class_12.t27

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+module damage_class_12
+
+// shape: [[][9,
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/trees/quadtree.t27:23
+// signals: doubled-bracket
+pub struct Damaged {
+    points : [[][2,
+}

bootstrap/tests/fixtures/damage/damage_class_13.t27

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+module damage_class_13
+
+// shape: [[9]?*X",
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/search/aho_corasick.t27:14
+// signals: odd-quote
+pub struct Damaged {
+    children : [[256]?*ACTrieNode",
+}

bootstrap/tests/fixtures/damage/damage_class_14.t27

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+module damage_class_14
+
+// shape: [[]X(X, X)",
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/collections/btree.t27:21
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    children : [[]BTreeNode(K, V)",
+}

bootstrap/tests/fixtures/damage/damage_class_15.t27

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+module damage_class_15
+
+// shape: [[]?X(X)",
+// 1 line(s) in the corpus share this shape
+// first seen: specs/tri/collections/skip_list.t27:15
+// signals: doubled-bracket,odd-quote
+pub struct Damaged {
+    forward : [[]?SkipNode(T)",
+}

bootstrap/tests/fixtures/terminator/eof_generic.t27

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+module eof_generic
+
+// End of file inside an unclosed generic argument list. The lexer returns Eof
+// for every subsequent call, so a recovery scanner that refuses to accept Eof as
+// a terminator has a stationary state on an infinite input, and the loop is a
+// consequence of the construction rather than a rare race.
+pub struct Hangs {
+    a : Map<K, Vec<V

bootstrap/tests/fixtures/terminator/eof_hazard.t27

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+module eof_hazard
+
+pub struct AtEof {
+    a : Map<K, V

bootstrap/tests/fixtures/terminator/field_swallow.t27

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+module field_swallow
+
+pub struct Trunc {
+    a : Map<K,
+    b : u8,
+    c : u16
+}

bootstrap/tests/fixtures/terminator/nested_types.t27

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+module nested_types
+
+pub struct Holder {
+    plain : u8,
+    generic : Map<Key, Value>,
+    tuple : (A, B),
+    nested : Vec<Vec<u8>>,
+    arr : [4]u16,
+    last : i32
+}
+
+pub fn use_holder(h : Holder) -> u8 {
+    return h.plain;
+}

bootstrap/tests/fixtures/terminator/semicolon_phantom.t27

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+module semicolon_phantom
+
+pub struct Semi {
+    a : Map<K, V;
+    b : u8
+}

bootstrap/tests/fixtures/terminator/tuple_type.t27

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+module tuple_type
+
+// A well-formed tuple type in a struct field. The comma inside the parentheses
+// is a type-argument separator, not a field terminator, and this fixture exists
+// so that a recovery change which mistakes one for the other is caught on input
+// that is not malformed at all.
+pub struct Pairs {
+    single : (A, B),
+    nested : (A, (B, C)),
+    with_generic : (Map<K, V>, u8),
+    last : u16
+}

bootstrap/tests/fixtures/terminator/unbalanced.t27

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+module unbalanced
+
+pub struct Broken {
+    a : Map<K,
+    b : u8
+}
+
+pub fn f() -> u8 { return 0; }