Break the loraDatasetCaption <-> loraDatasetGenerate static import cycle (#5917)

[atomantic]

Summary

Breaks the loraDatasetCaption.js <-> loraDatasetGenerate.js static ESM import cycle by extracting the shared subject-derivation helpers into a new leaf, server/services/loraDatasetSubject.js.

• New leaf owns loadDatasetSubject, extractSubjectSignaturePhrases, plus the small helpers they are built on (normalizeEntryKind, subjectLabel, flattenValue). Its only service import is getUniverse from universeBuilder.js, which imports no loraDataset* module, so no new ring is possible.
• loraDatasetCaption.js now imports the subject helpers from the leaf instead of from the generator — the Caption -> Generate edge is gone. The remaining Generate -> Caption edge (resolveCaptionModel / withCaptionVisionLock) is one-directional and acyclic, so no second leaf was needed.
• loraDatasetGenerate.js imports the moved helpers back from the leaf; its public API (generateDatasetImages, getDatasetVariationAxes, sliceReferenceSheet, prompt/crop builders) is unchanged.
• Deletes this component's entry from KNOWN_CYCLIC_COMPONENTS in serviceImportCycles.test.js (the suite fails on a stale entry, so the baseline only shrinks).

Closes #5917

Test plan

• npx vitest run services/serviceImportCycles.test.js services/loraDatasetGenerate.test.js services/loraDatasetCaption.test.js — 3 files, 60 tests, all pass (cycle ratchet confirms the component is gone and no new cycle appeared).
• npx vitest run routes/loraDatasets.pagination.test.js — 2 tests pass (route layer that mocks both modules still wires correctly).
• Local reviewer (mtplx, low effort): one finding (removed extractSubjectSignaturePhrases export from the generator without re-export). Verified repo-wide that no consumer uses the old path — routes import only generateDatasetImages / getDatasetVariationAxes / sliceReferenceSheet, and the one test that imported the moved symbol now imports it from the leaf — so no re-export shim was added.