promote FableLoom intent transitions to per-edge sub-resource routes

[atomantic]

Summary

• Intent transitions are now sub-resources under their scene node, so any writer adds or edits one edge per call instead of replaying the whole array off a snapshot another writer has already moved past:
  • POST /api/fableloom/:id/episodes/:episodeId/nodes/:nodeId/transitions → { loom, transition } (server mints the tr-* id)
  • PATCH …/transitions/:transitionId → the loom
  • DELETE …/transitions/:transitionId → the loom
• Back-compat is intact: the whole-array transitions key on the node PATCH is unchanged, so a client or federated peer from before these routes keeps working. No on-disk shape change, so no migration and no sync-payload version bump.
• The scene editor saves rows individually. A path is created server-side first, so every row the panel holds already carries its id — syncTransitionsFrom, the re-sync that pushed server-minted ids back into locally-added rows after each save, is gone. Blur-saves, the target select, and remove each touch exactly one edge.
• Supporting cleanups rolled in: runSave collapses the pending-save / error-toast plumbing shared by every write in the panel; a failed delete restores the row from the record instead of leaving a path that only looks gone; a blurred row with no matching record row now saves rather than silently dropping the edit (absent ≠ unchanged).
• Schemas (transitionCreateSchema / transitionPatchSchema) derive their caps from LOOM_LIMITS, and addNodeTransition refuses to exceed TRANSITIONS_MAX with 400 LIMIT_REACHED.

Closes #4786

Test plan

• server: full Vitest suite green (1563 files). New coverage in server/services/fableLoom/records.test.js (mint + read back the row, siblings untouched on add/delete, partial patch keeps the id and unlisted fields, 404 on an unknown id, the cap) and server/routes/fableLoom.test.js (dispatch args for all three routes, id in a POST body is stripped rather than honored, and the whole-array node PATCH still forwards untouched).
• client: full Vitest suite green (727 files). New client/src/components/fableloom/LoomNodeEditor.test.jsx asserts the add path goes through addLoomTransition and never updateLoomNode, a blurred edit PATCHes one row by id, an unchanged blur skips the round-trip, and remove DELETEs by id. apiFableLoom.test.js covers path building and segment encoding.
• node scripts/run-ci-lint.js clean.
• Manual: in the scene editor, add a path, retarget it, edit its intent and trigger phrasings, and remove it — each action is a single request against the one edge.