Pre-0.1 cleanup: publish compiled JS, tighten API, fix CI

[johnsoncodehk]

Cleanup ahead of the 0.1.0 release.

Package

• Publish compiled dist/*.js + .d.ts instead of raw .ts sources, so consumers no longer need a TS-capable runner (Node refuses to strip types inside node_modules). Built via prepack.
• files: ["dist"] instead of listing every source file.
• Add description, keywords, repository.
• Stop exporting detectCapabilities, hasEffectCleanup, hasComputedThrows — only used internally. Public API is now testSuite, SkipTest, expect, setExpect + types.

README

• Usage example skips type === "behavioral" sections (those return strings and never fail) and imports describe/test explicitly.

Tests / CI

• reatom adapter: cleanups run as AbortSignal listeners, so a throwing cleanup surfaced as 3 uncaught exceptions after tests finished. Now caught and logged; matrix results unchanged.
• npm test uses && instead of ; so vitest failures are no longer masked.
• New CI workflow on PRs: typecheck, vitest, build. Node 20 (EOL) → 22, npm install → npm ci.
• Rename test/frameworks/xReactivity.ts → solidSignals.ts (it adapts @solidjs/signals).
• Replace hand-written fs/path declarations with @types/node.
• Remove docs/coverage-matrix.md (internal working notes, partly stale).

Verification

• npm ci && npm run typecheck && npm test && npm run build pass; 0 unhandled errors; README results identical.
• npm pack → installed into a clean project, ran the full suite from plain JS with an alien-signals adapter: 178/178 pass. Types resolve under moduleResolution: nodenext.