[ETS] Preserve TypeScript structural object types

[CaelmBleidd]

What changed

• Preserve property-only object type literals as synthetic TYPE_LITERAL classes instead of degrading them to UnknownType. Field names, field types, modifiers, optional markers, and recursive references are retained.
• Preserve the TypeScript object keyword as an empty structural TYPE_LITERAL class.
• Emit each structural class only in its declaring file. Importing files reference the same source-based signature without emitting duplicate class definitions.
• Preserve generic structural aliases in declaration form and attach concrete type arguments at use sites. This also works when the type literal is reached through another alias, rather than appearing as the alias target directly.

Type literals containing unsupported or computed members still fall back to UnknownType as a whole, so the frontend does not expose a misleading partial shape.

Why this is needed

The native TypeScript frontend should preserve source type information without imposing runtime interpretation policy on downstream analyses. Previously, structural annotations lost their fields, imported structural types could be emitted more than once with the same signature, and generic shapes could be specialized by whichever use site happened to be lowered first.

The emitted EtsIR now has one stable, declaration-owned structural entity, while each use carries its own type arguments. Type-level anonymous structures (TYPE_LITERAL) remain distinct from runtime object literals (OBJECT).

Verification

• npm test: 212 frontend tests passed.
• npm run build: TypeScript checking, bundling, and runtime packaging passed.
• USVM integration against this JacoDB branch passed:
  • :usvm-ts:test;
  • :usvm-ts-dataflow:test.

[github-actions]

Test Results

  225 files  ±0    225 suites  ±0   16m 47s ⏱️ +5s
  776 tests ±0    764 ✅ ±0  12 💤 ±0  0 ❌ ±0 
2 046 runs  ±0  1 996 ✅ ±0  50 💤 ±0  0 ❌ ±0 

Results for commit e7f43e2. ± Comparison against base commit c4f57f0.

♻️ This comment has been updated with latest results.