feat: THR003 — throws {} on callback parameters (?bs 0.9)

[marcelofarias]

Summary

• Adds THR003: from ?bs 0.9, fires when a function-typed parameter declares throws { X } but the containing fn does not declare throws { X } in its own header.
• Closes the last leg of the callback-effect-leak trilogy: EFF002 (uses {}), EFF003/EFF004 (reads {}/writes {}), now THR003 (throws {}).
• throws {} annotations on callback parameter types are stripped from emitted TypeScript (no runtime impact).

Closes #73.

How it works

In parse-fn.ts, buildArgsTs already strips uses {}, reads {}, and writes {} from function-typed parameters and collects them into paramCaps/paramReads/paramWrites. This PR extends that same pattern to throws {}, adding paramThrows to FnDecl.

In eff-check.ts, the THR003 check mirrors EFF003/EFF004: if decl.paramThrows contains labels not present in decl.throws, emit THR003. Gated on ?bs 0.9.

Test plan

• pnpm -r build && pnpm test — 596/596 pass (9 new tests)
• Callback declares throws { NetworkError }, outer fn declares nothing → THR003
• Callback throws is a strict superset of outer fn throws → THR003 (missing types named)
• Multiple callback params each declaring throws → THR003
• Outer fn declares exact callback throws → clean
• Outer fn over-declares throws → clean
• Callback has no throws {} annotation → clean
• Below ?bs 0.9 → no THR003
• throws {} stripped from emitted TypeScript
• bs explain THR003 returns rule/idiom/rewrite
• MCP known-codes test updated

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a new compiler diagnostic THR003 (gated on ?bs 0.9) to prevent “callback throws” leakage: if a function-typed parameter declares throws { X }, the containing function must declare throws { X } (or a superset) in its own header. This extends the existing callback-surface checks (EFF003/EFF004) to exception surfaces and ensures throws {} on callback parameter types is stripped from emitted TypeScript.

Changes:

• Extend fn parsing to collect throws {} labels from function-typed parameters (paramThrows) while stripping them from emitted TS types.
• Add THR003 enforcement to passEffCheck (mirroring EFF003/EFF004) and register THR003 in error code registries + MCP explanations.
• Add focused compiler tests for THR003 and update MCP known-codes coverage.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
CHANGELOG.md	Documents THR003 behavior and TS-stripping in the ?bs 0.9 “Added” section.
packages/compiler/src/parser/parse-fn.ts	Collects callback-parameter throws {} into paramThrows and strips it from emitted TS argument types.
packages/compiler/src/passes/eff-check.ts	Implements THR003 check under the existing ?bs 0.9 gate alongside EFF003/EFF004.
packages/compiler/src/error-codes.ts	Registers THR003 metadata (rule/idiom/rewrite/example) for bs explain / diagnostics enrichment.
packages/compiler/tests/thr003-check.test.ts	Adds coverage for THR003 triggering/suppression, version gating, TS stripping, and diagnostic shape.
packages/mcp/src/explanations.ts	Adds MCP explanation content for THR003 (body + fails/passes examples).
packages/mcp/tests/server.test.ts	Updates MCP known-codes list to include THR003.

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