📝 Document the TypeScript 5.0 type floor

Consumers need TypeScript >= 5.0: the emitted declarations use const type
parameters and keep relative .ts specifiers, both of which resolve only on
TS >= 5.0. That also makes the emitted .ts specifiers a non-issue, so the
backlog item is closed without a .d.ts post-step: TS <= 4.9 cannot parse
the declarations anyway. The README/CONTRIBUTING wording now says the
rewrite applies to the JavaScript output, not to the declarations.

CONTRIBUTING.md

@@ -6,7 +6,7 @@ This document is for maintainers and contributors working on the project itself.
 
 CI and review will bounce these even though `npm run check` and the linters don't catch them. They're the high-frequency things a contributor (or an agent) reaches for by default:
 
-- **Source imports use `.ts` extensions, never `.js`.** `node --strip-types` resolves the `.ts` form at test time; `rewriteRelativeImportExtensions` emits `.js` in `dist/`. "Pre-fixing" an import to `.js` breaks the inner loop. (rationale: README § Tooling decisions)
+- **Source imports use `.ts` extensions, never `.js`.** `node --strip-types` only resolves the `.ts` form at test time; "Pre-fixing" an import to `.js` breaks the inner loop. (rationale: README § Tooling decisions)
 - **A new `npm run` script must reuse an existing prefix** (`create:` / `check:` / `fix:` / `test:` / `watch:` / `maintain:` / `publish:` / `setup:`). If none fits, that's a signal the script doesn't belong in the pipeline — not a reason to invent a new prefix. If it genuinely does belong, add the prefix to both lists here in the same commit; an undocumented prefix becomes invisible and quietly accrues members. (see [Script prefix convention](#script-prefix-convention))
 - **`oxlint-disable` directives live in source, not `.oxlintrc.json`.** The trade-off must sit next to the code it silences. This is a _human_ last-resort convention; agents must not add these — see [AGENTS.md § Never do](./AGENTS.md#never-do). (rationale: README § Tooling decisions)
 - **Don't put slow / network / whole-project scans in `check` or pre-commit.** Advisory scans are not correctness gates; they belong under `maintain:`. (see [Feedback tiers](#feedback-tiers) and [Script prefix convention](#script-prefix-convention))