refactor: strip comments and simplify the downgrader in orpc style

[dinwwwh]

Brings packages/ in line with the middleapi/orpc convention of letting names explain the code. The downgrader source and tests lose their file headers, internal JSDoc, SAFETY: notes, and inline restatements of spec rules the README already documents; the types package loses its file headers and vocabulary divider comments while keeping the per-field spec JSDoc that powers hover docs. Alongside, the downgrader drops a few layers of indirection. Behaviour is unchanged.

Simplifications

• convertSpec wrappers are inlined into downgradeSpecV31ToV30 and downgradeSpecV32ToV31, and the exported functions cast their result directly.
• Path Item $ref inlining merges the referenced item into the referencing one before converting, so the two casts and the spread-versus-assign note are gone; output is identical.
• The mutualTls set is replaced by an isMutualTls lookup on the existing scheme-type map.
• Literal-key writes use plain assignment instead of setOwn; required and convertType lose redundant branches.

Kept on purpose

• convertSchema stays as the untyped internal entry point: the recursion feeds it unknown subtrees, and routing those through the typed export would need casts at every call site or an unknown overload in the public API.
• @ts-expect-error directives in the type tests, and one-line notes on why two corpus fixtures are excluded.

Testing

• pnpm lint, pnpm type:check, and pnpm test pass; all 350 tests green with snapshots unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[pullfrog]

ℹ️ Clean refactor — one doc-preservation observation inline.

Reviewed changes

• Comment stripping — removed file headers, internal JSDoc, SAFETY: notes, and spec-rule restatements from the downgrader source/tests and the types package, keeping the per-field schema JSDoc that powers hover docs.
• convertSpec wrappers inlined — downgradeSpecV31ToV30 / downgradeSpecV32ToV31 now cast their result directly instead of routing through a private wrapper.
• Path Item $ref inlining rewrite — merges target and own before converting instead of converting-then-skipping; verified equivalent, and the own-wins / chain / callback / cycle / mutualTLS-inside-inlined behaviors are all pinned by existing tests (v3.1-to-v3.0.test.ts:158-297).
• mutualTls set → isMutualTls lookup — reads context.schemeTypes directly; the two are bijectively equivalent since mutualTls was populated for exactly the names whose resolved type is 'mutualTLS'.
• setOwn → plain assignment, branch dedups — every replaced site writes a literal key (type, $ref, minimum, required, …), never a user-dynamic key, so the __proto__ hardening in setOwn is genuinely unnecessary there; the required / convertType / applyTypes / convertContentKeywords restructurings are control-flow-identical.

I verified equivalence by tracing each restructure (the content: convertContent in convertParameterOrHeader is a pre-existing alias of the removed inline mapRecord(item, convertMediaType)), and confirmed correctness: 350 tests green with snapshots unchanged, output re-validated against the official 3.0/3.1 schemas, plus pnpm type:check and pnpm lint clean.

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[pullfrog]

The removed header was the only place documenting the cross-version architecture and version delta — nullable is gone (use type arrays), $ref is a plain schema keyword, boolean schemas are valid, and structurally-identical types are re-exported from ./v3.0. The per-field JSDoc survives and even mentions Draft 2020-12, but nothing anywhere now states the relationship/strategy this header captured, which the downgrader's SCHEMA_FIELDS/applyTypes implicitly encode. Consider keeping that one orientation paragraph — as a short header or in a package README — so the info isn't lost from the repo.