chore: add a typecheck gate and align TypeScript versions

[childrentime]

Problem

The repo had no working tsc invocation, and no typecheck script. Neither config was clean on main:

• packages/core/tsconfig.json → 7 errors
• root tsconfig.json → 258 errors, all from zod@4.4.3's .d.cts, which the pinned TypeScript 4.9.5 cannot parse

Underneath that sat a version split: root TypeScript was 4.9.5, packages/core's was 5.8.3. The two disagree about lib.dom — GeolocationCoordinates.toJSON exists in 5.8 but not in 4.9 — so an editor picking up the root TypeScript flags code that CI requires, and vice versa.

Changes

The gate

• typecheck (tsc --noEmit) on @reactuses/core, plus a root passthrough, wired into CI after lint.

The version split

• Root TypeScript ^4.9.5 → ^5.8.3, matching packages/core.
• typescript.tsdk → the workspace TypeScript, so editors and CI resolve the same lib.dom.

The 7 pre-existing errors

• useGeolocation: initCoord gains the toJSON that lib.dom now requires. It returns its own fields, so JSON.stringify output is unchanged; the constant is internal (not exported from src/index.ts).
• useOrientation: ScreenOrientation.lock() ships in browsers but is absent from lib.dom, which declares only unlock() — narrowed via a local interface rather than an any cast.
• Specs: two genuinely unused params prefixed with _; useMap's reset case widened past its as const literals, since that test adds a third key.

No runtime behavior changes.

Verification

pnpm typecheck, pnpm lint, pnpm build, and the full 307-test suite all pass. Full suite run 3× on this branch and 3× on main — see the note below.

Notes for review

1. An unrelated bug I did not touch. useOrientation's lock guards look inverted:

const lockOrientation = (type: UseOrientationLockType) => {
  if (isBrowser) {
    return          // isBrowser === true means we ARE in a browser
  }
  ...
  return window.screen.orientation.lock(type)
}

With isBrowser = typeof window !== 'undefined', this returns early in every browser — so lockOrientation / unlockOrientation are no-ops where they should work, and fall through to touch window during SSR. Neither is covered by index.spec.ts. Left alone deliberately: it's a behavior fix, not a type fix, and belongs in its own PR.

2. A flaky test, pre-existing. useDebounce/index.spec.ts failed twice under full-suite parallel load, both times immediately after a heavy install + build. It passes 5/5 in isolation and 3/3 on the full suite once the machine is quiet — and main shows 3/3 as well, so it is timing sensitivity rather than anything in this change. Worth watching once CI runs it on shared runners.

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com

[ostapondo]

Confirmed the inverted guard you mention in the description — isBrowser is
typeof window !== 'undefined', so lockOrientation / unlockOrientation
return early exactly when we are in a browser, and only ever reach
window.screen during SSR. Happy to pick this up as a follow-up PR with
tests, if nobody else is on it.

[ostapondo]

LGTM — went through the whole diff, turned up nothing. The typecheck scope
(src + global.d.ts + jest-setup, scripts excluded) makes sense.