feat(sum): named record as a sum-variant payload (M4)

[assapir]

Summary

A sum-type variant can now carry a previously-declared named record as its payload — e.g. Method = Get / Post(Body) where Body is a record. This is the confirmed M4 "authoritative composite types" gap and unblocks the HTTP client's Method/Request/Response (#143).

Scope is part (A) only (named record as a sum payload). Part (B) — a named composite as a record field — is intentionally deferred.

Step-0 state (before this change)

• Boxed = Box(Point) (single-variant) never parsed as a sum: a single-variant Name(Payload) is parsed as an expression, so it failed with "Undefined variable". Multi-variant sums are the real path (and match Method).
• A multi-variant sum with a record payload (Boxed = Box(Point) / Empty) failed type-checking at the declaration: error: Type mismatch: expected Num, got Named { name: "Point", fields: [] }. The checker rejected any non-scalar payload up front, and the parsed named payload carried empty fields.
• So the gap was in the checker (payload rejection + unresolved named type), with a matching codegen gap (a bound record payload was not registered as a record, so p.field errored "Field access not fully implemented").

What changed

• Checker (decls.rs): a sum declaration now resolves each variant payload. Built-in scalars (Num/Text/Bool/$) pass through; a named type must resolve to an already-declared record (its fields carried through). Arrays are rejected by the parser; a nested sum, an unknown name, and heterogeneous concrete types per position are rejected by the checker. The per-position consistency check runs over the resolved variants.
• Codegen (matching.rs): when a match binds a record-typed payload, it is registered like any record local (record_types + var_named_types), so field reads and method calls on the binding resolve. The payload rides in the tagged-union slot by pointer (the record ABI), consistent with the uniform layout — no new boxing path.

Preserved invariants

• Sum payloads stay "consistent type per position" (the position may now be a named record).
• Num/Text/Bool/$/array payloads and the Result boxing path are unchanged.

Ships with

• examples/nested_composites.ql — a Method = Get / Post(Body) sum: construct, match, read a Text field, call a method; self-asserting, exits 0 (JIT + AOT).
• Run tests (field read, method + Text field, nullary sibling by tag) and type-error tests (nested sum rejected, unknown name rejected) in tests/run_test.rs.
• docs/LANGUAGE.md sum-type section, feature matrix, and Known limitations updated.

Gate

cargo fmt --check, cargo clippy --all-targets -- -D warnings, and the full cargo test suite under RUSTFLAGS=-D warnings all pass.

PARKED — do not merge without explicit approval.

🤖 Generated with Claude Code