fix: stop older FableLoom peers erasing delivery plans and beat outlines (#6005)

[atomantic]

Summary

A FableLoom story synced from a federated peer running an older wire schema silently lost data. preserveLegacyVisualProduction retained render settings, protagonist bindings, and challenge mappings across a legacy whole-record LWW overwrite, but not the series delivery plan or the per-episode beat outline. A peer on fableLoom schema v3 or older cannot represent either field, so its sanitizer drops them and the merge wrote that gap back over the upgraded install's copy — erasing overnight/inter-episode voicemails, the next-season teaser, and every episode's outline plus its validation state.

The receiver now restores both from the local record when a <=v3 sender omits them:

• preserveLegacyDeliveryPlan puts deliveryOptions, interEpisodeVoicemails, and nextSeasonTeaser back, keying on the key being absent (hasOwnProperty) rather than empty, so a peer that understands the fields and clears them still propagates the clear.
• preserveLegacyStoryOutline restores localEpisode.storyOutline whole when the remote episode carries none, and otherwise keeps today's per-scene plotPointId/challengePhase merge. This also lifts the outline branch out of the episode map, which was the densest nesting in the function.

Scope notes (deviations from the filed plan, detailed in a comment on #6005)

Tracing when each field entered the wire schema changed two details of the proposed fix:

1. Guarded at senderVersion < 4, not < 5. Both the delivery plan and episode.storyOutline shipped in the commit that bumped fableLoom from 1 to 4, so a v4 peer round-trips them intact. Preserving at < 5 would resurrect a plan or outline a v4 author deliberately deleted. The real loss window is v1–v3, and that is what closes here. The existing < 5 handling of plotPoints[].kind, outline-scene plotPointId/challengePhase, and productionStatus is unchanged.
2. participationMode / audienceCommunicationMedium need no preservation. They predate wire v1, so no sender version fails to represent them — every v1–v3 peer returns them intact. Adding preservation would instead discard a legitimate participation-mode change made on an older peer. That acceptance criterion is closed as not-a-bug.

The v4 note in server/lib/schemaVersions.js now records the outline and delivery-plan fields, which had shipped undocumented and cost this fix an archaeology pass.

Test plan

• cd server && npm test -- services/fableLoom/records.test.js — 54 passed.
• cd server && npm test -- services/fableLoom/ services/sharing/ lib/schemaVersions — 32 files / 854 tests passed.
• New: preserves the delivery plan and beat outline when a v3 peer wins an unrelated LWW edit — verified red on main's merge logic before the fix, green after.
• New: lets a v4 peer clear the delivery plan and beat outline it fully understands — pins the version boundary at 4 so a future widening to < 5 cannot pass unnoticed.

Closes #6005

https://claude.ai/code/session_01VjkWVTfzKyRuAv3HEsspwN