feat: ship TypeScript declarations

[killerwolf]

P1, item 1 of the discoverability plan.

Why

No types meant every TypeScript consumer got an implicit any and had to write their own declarations or pick a different library. It is the first thing an evaluator checks, and npm surfaces it on the package listing.

What changed

• src/index.d.ts — hand-written, ~140 lines. Generating from the JSDoc produced a weaker surface than writing it directly, since the source is plain JS.
• package.json — types entry point plus a types condition first in the exports map, so both bundler and node16/nodenext resolution find it.
• npm run types:check — tsc --noEmit over types-test.ts, a compile-time exercise of the whole public surface including @ts-expect-error negative cases. Wired into the Code Quality workflow. Declarations that silently drift from the implementation are worse than no declarations.
• README — a short TypeScript section.

Two things the types make explicit

The docs previously only implied both:

1. Coordinates are always in the image's original pixels, not displayed pixels.
2. The style / handleStyle objects replace the defaults rather than merging into them — a partial override silently unsets every property it omits. Documented in the declarations; worth fixing in the implementation separately.

Verification

Beyond CI, I packed the tarball and installed it into a scratch project under moduleResolution: NodeNext. Types resolve, and getFocusPoint().x correctly types as number.

npm test (13 passing), lint:check, format:check and types:check all green. The declarations ship (3.8 kB); types-test.ts and tsconfig.json do not.

🤖 Generated with Claude Code