👷 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.

development/library.md

@@ -107,8 +107,8 @@ Each factory is two overloads whose order is load-bearing:
 - **Variance / `const` type parameters / `NoInfer` / `unique symbol` brands /
   defaulted type-param guards.** None change inference or evaluation order;
   `in`/`out` on the handler map broke contextual typing outright. `NoInfer`
-  specifically leaks into the emitted `.d.ts`, raising the consumer floor to
-  TypeScript 5.4 (README promises `>= 5.0`).
+  specifically leaks into the emitted `.d.ts`, which would raise the consumer
+  floor above the documented one (see [README § Requirements](../README.md#requirements)).
 - **Union merge**, **overload merge with only the exhaustive arm last**,
   **inferred universe**, **conditional `RequireKeys`**, **cases-first curried** —
   decided against while the API was single-object; their reasons (reported