test(types): restore TypeScript 6.0 to CI matrix

[joshuaellis]

Context

TypeScript 6.0 was provisionally added to the test-types CI matrix when expanding from ['5.0', '5.1', '5.2'] to broader coverage, but was dropped before merge because pnpm test:ts (tsc --noEmit) fails on it.

The repo currently develops against 5.7.2 and the matrix has been updated to ['5.0', '5.4', '5.7']. This issue tracks restoring 6.0 once the failures below are addressed.

Reproduction

pnpm install --frozen-lockfile
pnpm build-ci
pnpm add -w typescript@6.0
pnpm test:ts

tsc --version → 6.0.3.

Failure output

tsconfig.json(24,5): error TS5101: Option 'downlevelIteration' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
tsconfig.json(28,5): error TS5101: Option 'baseUrl' is deprecated and will stop functioning in TypeScript 7.0. Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
  Visit https://aka.ms/ts6 for migration information.

Both are tsconfig.json deprecation errors, not source-level. TS 6 removed a pile of legacy options; both need a positioning decision before silencing or removing.

Options to resolve

1. Drop the options. downlevelIteration is only needed when target < ES2015; the repo targets ES2020, so it can likely go. baseUrl is unused (no path-style imports rely on it that aren't already covered by moduleResolution: Bundler). Cleanest fix.
2. Add ignoreDeprecations: "6.0" to silence for now. Quick but kicks the can to TS 7.

Definition of done

• tsconfig.json clean of TS 6 deprecation errors.
• pnpm test:ts clean on 6.0 and still clean on the existing matrix versions.
• 6.0 added back to the matrix.ts array in .github/workflows/tests.yml.