👷 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.

CHANGELOG.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- add a CI `compat` job that type-checks the suite and a consumer fixture
+  against the minimum supported TypeScript (5.9), consuming the built `dist/`
+  and gating `publish`
+- correct the documented consumer floor to TypeScript >= 5.9 and drop the
+  unsupported `node10` resolution claim
+
 ## [0.8.3] - 2026-09-28
 
 - upgrade dependencies: oxfmt 0.71 (the only range widened), oxlint 1.86,