runtime: close out Wasmtime concurrency coverage gaps

[lannbot]

Summary

Complete the disposition of the remaining supplementary Wasmtime concurrency subset: fix executable runtime gaps, recognize narrowly proved diagnostic equivalents, and document compatible/native-only residuals.

Runtime

• Transport component-boundary traps with a native Wasm trap carrier so guest catch_all cannot intercept them. Exact carrier identities retain the original cause and physical/logical origin across nested and JSPI boundaries; no global pending-cause slot. Core exceptions caught within a component remain ordinary exceptions.
• Enforce unresolved synchronous-lift progress using callee-instance work, independent of unrelated host retention. Distinguish actual CM blocking from host-import latency and mandatory cancellation/start-call continuation.
• Offer pending host admission at safe canonical boundaries, preventing starvation behind perpetual callback YIELD work.
• Implement explicit thread creation, index, suspend, resume-later, resume/suspend/yield/promote operations using the existing Task/Thread scheduler. Preserve operand validation before cancellation, target-first transfer, logical task identity, original exceptional causes, and post-result thread lifetime.

Explicit limitation

thread.new-indirect validates ordinary canonical-final (i32|i64) -> () start types before execution. Valid structurally equivalent non-final/derived types may still be rejected. This is not spec-compatible full thread support: the current JS interface exposes nominal type tests rather than structural function-signature reflection. Complete support requires translator metadata/table instrumentation beyond this interface; the restriction and executable boundary tests remain tracked under #12.

Harness

• Add exact task-return diagnostic equivalents; retain the eleven future-write OR-message differences as explained compatible per-row divergences, not a global equivalence.
• Provide wasmtime/gc only to the destructor-context fixture as a real host boundary, matching its purpose with eager logical frames rather than pretending to force JS GC.
• Preserve native table-capacity test-control exclusion; add a 1,000-cycle cancellation/drop regression checking bounded slot reuse instead of a fake provider knob.
• Explain the massive-send exclusion as Wasmtime-specific transfer-fuel policy over an exponentially expanded value. No claim of equivalent aggregate allocation protection.
• End dedicated shell workers only after complete, synchronously written output. Validate header/done/file identities against the independent manifest; wire protocol regressions into just shells.
• Enumerate the observed Bun fn() unreachable diagnostic exactly, without broadening matching.

Final Wasmtime concurrency disposition

Starting async inventory:42 failures +4 skipped setups. Final:13 classified failures,0 skips:
-11 future-write diagnostic differences: spec-compatible rejection of non-IDLE operations.
-1 unavailable native table-capacity setup +1 dependent no-instance assertion; purpose covered separately by bounded-reuse regression.
-The excluded async massive-send file remains a documented native resource-policy test.

All executed async exception and explicit-thread cases now pass. Residual file/line keys remain in harness/src/wasmtime-expectations.ts; reasoning is in harness/README.md and contracts/intrinsics.md.

Verification

• Independent exception, thread/sync scheduling, and final harness reviews: LGTM. Real-Wasm regressions cover nested/same-store/cross-store trap delivery, suppressed carrier recovery, switches, cancellation under sibling exclusivity, post-result children, and start-type rejection without execution.
• Full just gates < /dev/null: PASS, /tmp/opencode/concurrency-closeout-final-gates.log.
• Runtime1072passed/2existingignored; seeded1069passed/5existingignored. New shell protocol gate6passed, then wired into the recipe and rerun directly.
• Official corpus:1511commands,1506executed,1468passed,38xfail,0runtime/capability skips,5unsupported directives; seeded1465passed with existing three-command deterministic-profile omission.
• Supplementary Wasmtime:607passed,69known failures,0skips,174excluded commands,0unexpected. Public guests pass FIFO/seed1.
• Required shell/browser/OPFS, packaging and available consumer smokes pass. JSC arm64 unavailability and declared not-applicable/translation-only smoke cases remain. Actual consumer git statuses unchanged before/after full verification.

No convention goldens, manifests, protocol marks, or plan format version changed. Automerge will be armed and watched through delivery. Related tracking: #12, #372.