An open universe (`string`, `number`, a template literal) makes the
handler map index-like, so a partial object satisfied the exhaustive
overload and the runtime `dispatch` throw was reachable. A universe
containing a member and its stringification (`"true" | true`, `1 | "1"`)
collapsed two members onto one key.
- reject broad universes and value/stringification collisions at the
factory, with a diagnostic naming the reason
- invert `PatternKey` against the universe (`Member<T, K>`) instead of
`PatternParam<K>`, so a standalone `"true"` is typed `"true"`
- intersect `UniverseGate<T>` into the handler and fallback parameters,
preserving `R` inference and the popup
A `boolean` / `null` / `undefined` discriminant cannot key a handler
map, so the tagged-union matcher now routes it through the
`PatternKey` / `PatternParam` projection the primitive-union matcher
already used. Both matchers share the projection from
`matcher-shared.ts`; `MapTaggedUnion`, `Handlers`, `HandledMembers`
and `MustBePartial` key off the projected form and invert it to
recover the real member.
Covers exhaustive and fallback dispatch, the redundant-fallback
guard, and the LSP popup offering the tags by name.
`getMatcher` / `getMatcherW` become `getPrimitiveUnionMatcher` /
`getPrimitiveUnionMatcherW`; `src/primitive.ts` and its test move to
`src/primitive-union.*`. The `Union` suffix mirrors `getTaggedUnionMatcher`.
tmu