Threads corpus: separate JSC WebAssembly refusal from zig-js worker support

The vendored PR-249 api/wasm-refused-sd7.js test requires every WebAssembly entry point to throw TypeError on a spawned shared-realm Thread. That is a JSC v1 policy, not a portable correctness rule, and it conflicts with zig-js intentionally supporting WebAssembly from Threads. CI run 29648482270 exposed the stale allowlist entry after worker WebAssembly became operational.\n\nAcceptance criteria:\n\n- [x] Remove the incompatible refusal case from the promoted green allowlist without editing the pinned vendored source.\n- [x] Classify the exact policy mismatch in the reference audit.\n- [x] Add native zig-js coverage for validate, Module, Instance, and execution inside a shared-realm Thread.\n- [x] Exercise the supported contract in parallel and serialized/GIL modes.\n- [x] Update all promoted/reference-only counts and thread documentation.\n- [x] Keep the focused unit and reference-audit gates green.

Implemented in f42b1bb. Exact Zig 0.17.0-dev.1417 focused unit coverage passes with module validation, compilation, instantiation, and execution in both GIL modes; the reference audit is green and reports 235/259 compatible promoted cases with the refusal policy explicitly classified.