Fix recursive union flattening

[schani]

Summary

• make flattenUnions recursively unify nested unions while memoizing normalized union member keys to close cycles
• add a high-level code comment explaining why the local normalized-union memo is needed
• preserve the old fallback for nested intersections so intersection resolution can still handle them in a later pass
• add the issue Does not generate from opds2 schema #2187 minimized schema as a normal schema fixture input, with a JSON sample
• skip that regression schema for Kotlin/Kotlin-Jackson, cJSON, C++, Elixir, and TypeScript-Zod schema fixtures because those fixtures already have limitations around top-level primitive/union/recursive alias cases
• the plain TypeScript schema fixture still runs the schema and sample

Fixes #2187
Fixes #1376

Verified real-world crash schemas

These are the two large schemas from #2890 that originally motivated this crash investigation. Both now generate successfully with this PR:

• Fixed Does not generate from opds2 schema #2187 / OPDS 2.0 feed schema: node dist/index.js --src-lang schema https://raw.githubusercontent.com/opds-community/specs/master/schema/feed.schema.json --lang typescript --out /tmp/qt-issues/opds.ts
• Fixed Maximum call stack size exceeded when generating Rust from large FHIR JSON Schema #1376 / FHIR schema: node dist/index.js --src-lang schema /tmp/qt-issues/fhir.schema.json --lang rust --out /tmp/qt-issues/fhir.rs

Validation

• npm run build
• QUICKTEST=true FIXTURE=schema-typescript,schema-typescript-zod npm test -- test/inputs/schema/recursive-union-flattening.schema
• QUICKTEST=true FIXTURE=schema-kotlin,schema-kotlin-jackson npm test -- test/inputs/schema/recursive-union-flattening.schema
• QUICKTEST=true FIXTURE=schema-cjson npm test -- test/inputs/schema/recursive-union-flattening.schema
• QUICKTEST=true FIXTURE=schema-cplusplus npm test -- test/inputs/schema/recursive-union-flattening.schema
• QUICKTEST=true FIXTURE=schema-elixir npm test -- test/inputs/schema/recursive-union-flattening.schema attempted locally but blocked because mix is not installed; skip verified by inspection and CI will validate with Elixir installed
• node dist/index.js --src-lang schema /tmp/bug.min.schema --lang typescript --no-render
• node dist/index.js --src-lang schema /tmp/bug.schema --lang typescript --no-render
• QUICKTEST=true FIXTURE=typescript npm test -- test/inputs/json/samples/pokedex.json
• node dist/index.js --src-lang schema https://raw.githubusercontent.com/opds-community/specs/master/schema/feed.schema.json --lang typescript --out /tmp/qt-issues/opds.ts
• node dist/index.js --src-lang schema /tmp/qt-issues/fhir.schema.json --lang rust --no-render
• node dist/index.js --src-lang schema /tmp/qt-issues/fhir.schema.json --lang rust --out /tmp/qt-issues/fhir.rs

C# schema-output comparison vs master

Generated C# for all 65 schemas under test/inputs/schema using both this PR and master.

• PR: 65/65 schemas generated successfully.
• master: 64/65 schemas generated successfully; recursive-union-flattening.schema fails with the original stack-overflow bug.
• Among the 64 schemas generated by both branches, 63 outputs are byte-for-byte identical.
• The only existing-schema output difference is vega-lite.schema: this PR collapses a duplicate recursive SpecClass/Spec split into a single recursive Spec type, plus removes one merged doc-comment fragment.

C# JSON-output comparison vs master

Generated C# for all 210 JSON files under test/inputs/json using both this PR and master.

• PR: 210/210 JSON inputs generated successfully.
• master: 210/210 JSON inputs generated successfully.
• All 210 generated C# outputs are byte-for-byte identical between this PR and master.