👷 Type-check the TS floor in CI

Add a `compat` job that type-checks the whole suite and a consumer fixture
against the minimum supported TypeScript (5.9), reusing the `dist/` artifact
`build` produced and gating `publish`. The compiler is resolved by npx, so it
never enters `devDependencies` or the local `check`/`verify` loop.

The fixture imports the package by name, resolving the emitted declarations
through the `exports` map; `expectTypeOf` / `.not.toBeAny()` make it reject an
`any`-typed declaration, which a bare compile would accept.

Correct the README consumer floor from >= 5.0 to >= 5.9 (set by `type-fest`)
and drop the `node10` resolution claim, which the exports-only entry never
satisfied.

compat/tsconfig.json

@@ -0,0 +1,15 @@
+{
+    "extends": "@tsconfig/strictest/tsconfig.json",
+    "compilerOptions": {
+        "lib": ["es2024"],
+        "module": "nodenext",
+        "target": "es2024",
+        "types": ["node"],
+        "skipLibCheck": false,
+        "noEmit": true,
+        "allowImportingTsExtensions": true,
+        "verbatimModuleSyntax": true
+    },
+    "include": ["../src", "../scripts", "*.ts"],
+    "exclude": ["../src/doc-test"]
+}