build: make theme generation incremental and bounded

[rwang23]

Summary

• replace all-at-once generation with a bounded 1–4 worker pipeline that reconciles one theme at a time through atomic writes
• make source-art provenance and every declared visual input part of a versioned render fingerprint, with safe v4-to-v5 migration and partial-generation guards
• add cache-aware progress, targeted/check modes, benchmark scenarios, schema validation, and documentation for the reproducible workflow

Verification

• npm run check:full
• npm run generate:benchmark -- --scenario=warm --max-ms=60000
• npm run generate:benchmark -- --scenario=one-theme --ids=qinglan-odyssey --concurrency=1 --max-ms=60000
• independent remediation review

Closes #4

[rwang23]

Superseded by #26, now merged into main at e2ed6e8. This branch head is already included in main; closing the draft avoids duplicate review and merge work.