surface linked FableLoom branching narratives inside series management

[atomantic]

Summary

Surfaces FableLoom looms inside series management at the presentation/cross-linking layer. Per the issue's decision, a loom stays its own record — no seriesType: 'branching' enum on pipeline_series, no pipeline_series schema change, no schema-version bump.

• GET /api/fableloom?seriesId= scopes the index to one series' looms (loomListQuerySchema in server/lib/fableLoomValidation.js, filtering in listLoomSummaries). A blank value is a no-op filter rather than a 400, so a caller can build the query from a possibly-unset id; a dangling id matches nothing instead of throwing.
• Series detail page gains a "Branching narratives" card (SeriesLoomsPanel) listing linked looms with their episode/scene/ending counts and a link into /fableloom/:loomId — mirroring how CatalogCastPanel surfaces linked catalog records.
• "New branching narrative" action mints a loom pre-linked to the series and its universe, then lands in the editor.
• Series index badges each row with its linked-loom count. A failed loom fetch degrades to "no badges", never a broken list.
• Loom editor header gains a series backlink. The ref is soft in both directions: deleting a series is never blocked, and an unresolvable seriesId renders no chip rather than a dead link.

Test plan

• cd server && npm test — full suite green (32,759 passed). New coverage: listLoomSummaries series scoping + dangling-id case (services/fableLoom/records.test.js), and the route's filter / blank-value / over-long-id cases (routes/fableLoom.test.js).
• cd client && npm test — full suite green apart from two known load-related flakes (a11yConventions timeout, Catalog.test.jsx act warning) that pass in isolation and are untouched by this branch. New suites: SeriesLoomsPanel.test.jsx, Pipeline.loomBadge.test.jsx, FableLoomStory.test.jsx (backlink present / deleted-series graceful / unnamed-series fallback / standalone), plus apiFableLoom.test.js query-encoding cases.
• cd client && npm run lint — clean.

Closes #4785