build: bundle declarations on typescript@6, type-check on native typescript@7

Dependabot #7 bumped typescript to 7.0.2 (the native Go compiler), which has no
programmatic Compiler API yet, so tsup's bundled-dts step (rollup-plugin-dts)
crashed and `pnpm build` broke.

Adopt the supported side-by-side setup:
- typescript@6.0.3 (JS-based, same language level) keeps the Compiler API that
  tsup --dts needs. ignoreDeprecations '6.0' is set on the dts step alone, so
  the baseUrl-removal bridge error is silenced without touching the shared
  tsconfig read by the native type-checker.
- @typescript/native-preview provides `tsgo`; check-types runs on the native
  compiler.

Restores the bundled, self-contained index.d.ts / index.d.cts (no unbundled
files, no post-build hacks). publint clean; attw green for node10, node16
(CJS+ESM) and bundler.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: chrisdengso

package.json

@@ -44,7 +44,7 @@
   "scripts": {
     "build": "tsup",
     "prepack": "pnpm build",
-    "check-types": "tsc --noEmit",
+    "check-types": "tsgo --noEmit",
     "lint": "eslint .",
     "format": "prettier --write .",
     "format:check": "prettier --check .",
@@ -65,13 +65,14 @@
     "@changesets/cli": "^2.29.7",
     "@eslint/js": "^10.0.1",
     "@types/node": "^26.1.1",
+    "@typescript/native-preview": "7.0.0-dev.20260707.2",
     "@vitest/coverage-v8": "^4.1.10",
     "eslint": "^10.7.0",
     "globals": "^17.7.0",
     "prettier": "^3.6.2",
     "publint": "^0.3.14",
     "tsup": "^8.5.0",
-    "typescript": "^7.0.2",
+    "typescript": "^6.0.3",
     "typescript-eslint": "^8.64.0",
     "vitest": "^4.1.10"
   },