✅ Add expect-type for type-level tests

- Install expect-type@1.4.0 (devDependency)
- New src/index.test.ts exercises the public API with both runtime
  assertions (node:assert/strict inside node:test) and type-level
  assertions (expectTypeOf().toEqualTypeOf, toMatchTypeOf, toHaveProperty)
- Tests are excluded from tsc -p tsconfig.build.json (no .d.ts pollution
  in dist/)
- Type-level checks happen during tsc --noEmit (run by 'npm run check' and
  the test script); expectTypeOf assertions have no runtime side effect,
  so they pair naturally with node --test in the same test() blocks
- cspell: add EDITMSG to dictionary (used in commit-message-template path)
- oxlint test-file overrides: silence no-nodejs-modules (we use node:
  builtins) and no-magic-numbers (literals in tests are fine)

Coverage:
- match() returns a builder with with/exhaustive/otherwise
- P.literal narrows to its literal type, P.type to its typeof target
- exhaustive() returns the union of handler return types
- otherwise() falls back when no case matches
- exhaustive() throws when no case matches

cspell.json

@@ -12,7 +12,8 @@
         "nocheck",
         "oxfmtrc",
         "oxlintrc",
-        "sortpackagerc"
+        "sortpackagerc",
+        "EDITMSG"
     ],
     "ignorePaths": ["dist", "node_modules", "public", "coverage", "*.svg"]
 }