Harden runtime morph authoring

[meekmachine]

Summary

• preflight runtime morph target batches before mutating meshes
• reject relative/absolute morph target mode mixing on existing morph geometries and within a batch
• refresh morph caches after any partially applied batch if an unexpected error occurs
• add regression coverage and a recent-changes note

Context

This follows the architecture review for LoomLarge #619 and the merged runtime morph authoring work in #123. LoomLarge currently sends relative deltas, so this hardens the public Loom3 API before broader external or AI-authored morph payloads rely on it.

Validation

• npm ci
• npm test -- --run src/engines/three/Loom3.morphAuthoring.test.ts
• npm run typecheck

[meekmachine]

Cross-repo context:

• Follows the review findings from merged runtime morph authoring PR Add runtime morph target authoring #123.
• Supports LoomLarge authored morph persistence/replay in meekmachine/LoomLarge#619.
• Complements the Meshes-tab bicep/body recipe path now added to LoomLarge #619.

The important runtime guarantees added here are: no mixed relative/absolute morph modes on one geometry, batch preflight before mutation, and cache refresh after any unexpectedly partial batch application.

[meekmachine]

Coordinator check: #160 is non-draft, clean/mergeable, has no unresolved review threads, and Build, Test, and Typecheck are passing on head e40110a. Scope is bounded to runtime morph authoring hardening in src/engines/three/Loom3.ts, src/engines/three/Loom3.morphAuthoring.test.ts, and RECENT_CHANGES.md, and it is the current Loom3 dependency for LoomLarge #619. Classified ready-for-human-merge; no implementation agent assigned.