🐛 Reject mixed literal and broad universes

`UnsupportedReason` tested `IsLiteral<PatternKey<T>> extends false`.
`IsLiteral` is `boolean` for a union that mixes a literal with a broad
type (`"a" | \`x-${number}\``), so `boolean extends false` is false and
the mix was accepted. Test `extends true` instead, which rejects it.

Found while adding property-union broad-universe coverage; the gate is
shared with the primitive-union matcher.

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- reject a universe that mixes a literal with a broad type (e.g.
+  `"a" | \`x-${number}\``), which the finite-literal gate let through
 - narrow the tagged-union matcher's handler parameters for a union-valued or
   optional discriminant, and its fallback to the unhandled tags, instead of
   passing `never`