fix(core): honor --no-combine-classes for structurally identical classes

[schani]

The bug

--no-combine-classes had no effect for a specific case reported in #1265: when two JSON objects at different paths in the input have exactly the same shape (same property names and types), quicktype always merges them into one class, regardless of the flag.

Example: given

{
  "amount": { "initialValue": 1000000.0 },
  "rate": { "initialValue": 0.012 }
}

--no-combine-classes and default output were byte-identical — only one Rate class was ever generated for both amount and rate.

Root cause

--no-combine-classes is correctly wired for its documented purpose — disabling the CombineClasses rewrite pass, which merges classes with overlapping-but-not-identical properties (verified this works correctly with a non-identical-overlap test case; it toggles between merged/separate as expected).

The reported case is different: classes that are exactly, structurally identical. Those get deduplicated much earlier, in TypeBuilder.getClassType (via classTypeIdentity), during raw JSON inference (input/Inference.ts's TypeInference.inferClassType) — long before the optional CombineClasses rewrite pass, gated by the flag, ever runs. So there was no code path left for the flag to affect in this case.

Note CombineClasses.ts's tryAddToClique already has, as an unconditional first step, a check that merges exact structural duplicates — this is what makes it safe to stop deduplicating at the identity-cache level for this case: when combineClasses is left on (default), CombineClasses.ts re-merges these classes anyway, so default output is unaffected.

The fix

Thread combineClasses through Run → InputData → JSONInput → TypeInference, matching the existing inferMaps/inferEnums plumbing. In TypeInference.inferClassType's non-fixed-class branch, when combineClasses is false, tag the class's type attributes with a new TypeAttributeKind whose requiresUniqueIdentity returns true — an existing extension point (already used for StringTypes) that makes TypeBuilder's identity-based type cache skip deduplication for that type, so it survives as a distinct class through inference and later graph rewrites instead of being silently unified with same-shaped classes from other paths.

This only affects classes built from JSON inference (getClassType), not JSON-Schema-derived classes (getUniqueClassType/isFixed: true), matching combineClasses's own doc comment ("This doesn't apply to classes from a schema, only from inference").

Test coverage

• Added test/unit/combine-classes.test.ts, calling the quicktype() library API directly with combineClasses: false on JSON equivalent to the issue's sample and asserting both Amount and Rate classes are generated. It also protects the positional fixedTopLevels argument for custom Input implementations.
• Added the focused csharp-no-combine-classes end-to-end fixture. It runs the existing combined-enum.json sample with the global inference flag disabled, generates separate Foo and Bar classes (and their separate enums), and participates in the normal C# compile/round-trip fixture group. This also makes the option's generated output visible to the output-difference workflow.

Verification

• npm run build passes.
• Full unit suite: 171/171 tests pass (npx vitest run test/unit/).
• CLI repro from the issue: --no-combine-classes now generates a separate Amount class in addition to Rate; default output is confirmed byte-identical (md5-matched) to before the fix, for both the issue's sample and a separate near-duplicate-overlap sample used to confirm the pre-existing "similar classes" merging behavior is untouched.
• QUICKTEST=true FIXTURE=golang script/test, FIXTURE=python, and FIXTURE=typescript all pass (47/47, 62/62, 79/79) with default settings — no regressions.
• C# fixtures were checked for generation only; full round-trip execution wasn't available in this environment (dotnet not installed) — CI will cover full C# execution.

Fixes #1265

🤖 Generated with Claude Code

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