The transpiler was lowering `m.get(&k)` on `std::collections::BTreeMap`
(and `HashMap`) through the slice-style `rusty::get(container, idx)`
helper, which returns an `Option<element-by-index>` instead of
`Option<&V>`. The consuming match arm then fails to compile (clang
suppressed-error: "no viable conversion from `const rusty::BTreeMap<...>`
to function return type `int32_t`"), and any callsite that destructures
the wrong Option becomes unreachable for the @safe checker.

Changes:

* transpiler/src/codegen.rs: gate `.get`/`.get_mut` slice-style lowering
  on `!receiver_is_hashmap_like_expr` so map receivers fall through to
  regular method-call emission, which routes to `rusty::BTreeMap::get(K)
  -> Option<V&>` directly. Adds 3 codegen unit tests covering BTreeMap
  and HashMap `.get`/`.get_mut`.

* src/main.rs: when analyzing a file marked
  `// Auto-generated by rusty-cpp-transpiler`, drop the in-module
  `rusty::*` runtime functions before analysis — those live in the
  rusty/* headers, are noise for the checker on transpiler-generated
  modules, and previously masked any real fixture-side violation behind
  ~250 lines of header-internal findings. Adds two `is_*` helpers and
  two unit tests.

* src/parser/ast_visitor.rs: replace `SourceRange::tokenize()` with a
  thin source-slice lexer (`lex_cpp_token_spellings`) that returns
  `Vec<String>` spellings. libclang's `tokenize()` returns invalid
  token buffers for some generated module ranges and crashes the
  checker on the transpiled `.cppm` output; the direct-slice path
  side-steps that. Call sites are updated to the new return type.

* tests/transpile_tests/btreemap: minimal Rust fixture exercising
  `BTreeMap::new`, `insert(K, V) -> Option<V>`, and `get(&K) ->
  Option<&V>`. Built as a no-workspace `[lib]` so the parent workspace
  doesn't try to absorb it.

* tests/transpile_tests/run_btreemap_check.sh (mode 0755): end-to-end
  harness — builds both binaries, transpiles the fixture, injects
  `// @safe` annotations on the three exported functions, and requires
  a clean `rusty-cpp-checker` exit on the annotated output.

* .gitignore + tests/transpile_tests/.gitignore: track the new fixture
  assets and ignore the harness work dir (`.rusty-btreemap-check/`).

Verified from a clean state of this branch — see the published
scratchpad `kind:"diff"` record for the rerun results.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>