`Record<K, U>` where `U` is a union (flattens to >1 alternative) now
emits one binding carrying the original union, instead of one binding
per arm. The arms each lower to a distinct `Object<JsString>` /
`Object<Number>` / `Object<Boolean>` whose phantom is a compile-time
tag the runtime never sees, so the cartesian-expanded variants were
pure API-surface noise — every caller had to pick one of N siblings
backed by the same JS object.

Folding the union back at `flatten_type` time leaves the existing
JsValue-erasure path in `to_syn_type` to lower `Record<K, Union>` to
plain `&Object`. Single-typed Records (`Record<string, string>`)
still emit `&Object<JsString>` — the typed phantom is meaningful when
the value side is monomorphic.

Snapshot impact: workers-types' `WorkerLoaderWorkerCode.modules` (2
arms → 1), `GatewayOptions.metadata` (5 → 1), `VectorizeVector.metadata`
(7 → 1) collapse to single bindings. Auto-gen suffix logic also picks
the param-name-derived suffix (`_with_context`) instead of the
type-derived one (`_with_record`), since there's no longer ambiguity
to disambiguate against.

Replace the Record-only carve-out with a uniform LUB-based collapse
applied at every `flatten_type` recursion into a generic container's
element type. Reuses the existing LUB infrastructure
(`subtyping::lub_types`) that already drives `TypeRef::Union`
lowering and @throws aggregation, so there's one rule for "what does
a union of types mean at the binding layer" rather than two.

The `Record`-only `vs.len() > 1` special case is replaced by a
`lub_collapse` helper applied uniformly to `Array`, `Set`,
`Promise`, `Record`, and `Map` element types.

Implementation:

* `typemap.rs`: rename private `union_lub` → `pub(crate) lub_named`,
  the existing helper that already implemented exactly this LUB
  resolution. Doc-comment updated to describe both call sites.
* `signatures.rs`: new `lub_collapse` 4-line wrapper that calls
  `lub_named` and falls back to the original unflattened TypeRef
  when no meaningful LUB exists. Each generic-container arm of
  `flatten_type` calls it on the inner alternatives.

Snapshot impact (workers-types.rs only): 352 lines of phantom-sibling
explosions removed. `Record<K, Union>` collapses (already in wasm-bindgen#13)
preserved; new collapses across `Array<Union>`, `Promise<Union>`,
`Set<Union>` apply the same rule. Where the union members share a
named ancestor (e.g. `TypeError | RangeError` → `Error`), the
phantom narrows to that ancestor instead of erasing.

CONVENTIONS:

* Drop the Record-only carve-out subsection.
* Extend `## Subtyping LUB across unions` to mention that the same
  rule runs inside generic containers at flatten-time.

Tests: 8 new unit tests in `codegen::signatures::tests` covering
`Record`, `Array`, `Map`, `Promise` collapse plus direct
`lub_collapse` happy path / fallback / pass-through. 127 unit + 1
snapshot pass, clippy + fmt clean.