t27b: an array length written as a constant expression (Closes #7347)

[gHashTag]

Closes #7347. Refs #6063.

What

t27b refused an array type whose length is not a bare literal or a bare constant name (type [N]T), for example [W*H]u8 or [N]u8 where const N = H * 5. The reference compiles these, so the refusal blocked files the reference passes.

t27b now folds the length at compile time: integer literals, module constants (typed or untyped, including one defined by another expression), + - *, the wrapping forms, parentheses and unary minus. A length that does not fold to a non-negative integer is still refused: a division, a call, a run-time value, a negative length, or a typed constant that overflows its type. Typed module constants fold the same way, so const R = Q +% 10 with Q: u8 = 250 is 4. const OV = B + 100 with B: u8 = 200 is refused, matching the reference's refusal.

Evidence (lab, --jobs 2, no interpreter fallback, no skipped tests)

• Dogfood spec specs/tri/t27b/conformance/array_len_expr.t27: 5 pass, 0 vacuous in the reference; the same 5 in t27b, 16 runtime asserts.
• 7 mutants each fail the same tests in both. Of 22 probes, 9 pass in both, 2 fail in both, and 8 are blocked by the reference and refused by t27b. The other 3 are honest t27b refusals where the reference passes: a length using /, a length using a call, and a 50 KB local array over the frame limit.
• cargo test -p t27b: 14 suites, 128 passed, 0 failed (at 7a7688c).
• Lab corpus, branch 8c677e1 vs master e45ba56: t27b pass where the reference passes 818 -> 819, mismatch 0, reference_disagree 0. The only other change is routes/memory.t27, which now stops at FnDecl(frame size) instead of type [N]T.
• Ledger docs/reports/t27b_expectations.json recounted after merging master 05e633d (pass 534, vacuous 262, not_pass 25); no duplicate paths.

Foreign code

cli/t27b/src/lower.rs and cli/t27b/tests/source.rs are Rust. The owner approved foreign Rust in cli/t27b for #6063 and asked agents to set the owner-approved-foreign label themselves. Both files are listed in tools/policy/foreign-exceptions.txt.

NOW entry: docs/now/2026-10-07-t27b-array-len-expr.md.

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