WebAssembly profiles: exceptions and tail calls

Parent: #142 Depends on: #262 Roadmap: #134

Implement pinned exception-handling and tail-call profiles with exact JS/Wasm boundaries.

Acceptance:

• Pin exact proposal revisions and opcode/type inventories.
• Implement decoding, validation, execution, traps/unwinding, tags/exceptions, and tail-call stack behavior.
• Preserve JavaScript exception identity and WebAssembly exception typing across imports/exports.
• Keep tail calls stack bounded and compatible with precise GC roots.
• Publish upstream profile totals with no hidden exclusions.

Child issue map

• #288 — tail-call opcode/file inventory, decoding, and validation
• #289 — bounded tail-call execution frames and precise GC roots
• #290 — exception tags, binary IR, validation, and core unwinding
• #291 — JavaScript Tag/Exception API and cross-boundary identity
• #292 — exact upstream corpora, bounded CI, host matrix, and terminal evidence

Implementation order

#288 precedes #289, and #290 precedes #291. The tail-call and exception tracks may proceed independently once their inventories lock the exact pinned revisions. #292 closes only after all four implementation slices are complete and every declared upstream command has an explicit execution mode and result.

Tail-call binary/validation (#288) and bounded execution/root safety (#289) are complete. The remaining tail work is terminal proposal-corpus scoring in #292; exception implementation proceeds through #290 and #291.

Core exception handling child #290 is complete through e16c4f6: exact inventory, tags, modern IR/validation, typed unwinding, precise roots, allocation rollback, deep nesting, and normal/TSan evidence. Parent completion still correctly waits on #291 for the JavaScript identity boundary and #292 for exact proposal corpus totals/CI.

#291 is complete: the pinned JavaScript Tag/Exception API, JS/Wasm identity transport, precise-GC ownership, failure teardown, and equivalent JSC differential are now implemented and documented. The only remaining child is #292, which owns exact no-hidden-filter corpus totals and terminal host/sanitizer evidence.

All five child slices are complete. The final #292 inventories account for all 205 declared exception/tail-call commands with 192 public-API passes, 13 explicit text-parser N/A results, and no failures or runner errors. README and WebAssembly status now publish the exact totals, pins, reproduction, host boundary, sanitizer evidence, and JSC comparison.