✅ 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

package-lock.json

@@ -15,6 +15,7 @@
                 "c8": "^10.1.3",
                 "check-outdated": "^2.13.0",
                 "cspell": "^8.19.3",
+                "expect-type": "1.4.0",
                 "lefthook": "^1.11.12",
                 "oxfmt": "^0.66.0",
                 "oxlint": "^1.81.0",
@@ -2277,6 +2278,15 @@
                 "node": ">=4"
             }
         },
+        "node_modules/expect-type": {
+            "version": "1.4.0",
+            "resolved": "https://registry.npmjs.org/expect-type/-/expect-type-1.4.0.tgz",
+            "integrity": "sha512-KfYbmpRm0VbLjEvVa9yGwCi9GI34xvi7A/HXYWQO65CSD2u3MczUJSuwXKFIxlGsgBQizV9q5J9NHj4VG0n+pA==",
+            "dev": true,
+            "engines": {
+                "node": ">=12.0.0"
+            }
+        },
         "node_modules/fast-equals": {
             "version": "5.2.2",
             "resolved": "https://registry.npmjs.org/fast-equals/-/fast-equals-5.2.2.tgz",