The exact-equality `toEqualTypeOf` assertions already reject `any`, so the
`.not.toBeAny()` guards were redundant. Refresh the fixture header and
development/ci.md to match.
Give the primitive-union handlers a parameter and assert its member type, and
assert the tagged-union widening handler's parameter too. A matcher without
handler-parameter assertions cannot catch a wrong or `any` handler parameter:
the matcher's own `(shape: T) => R` type is built from `T` and the handler
return, so it stays correct even when `Member<T, K>` inversion is broken.
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.
tmu