Add character model caching and template cloning for createCharacter

[tracygardner]

Motivation

• Reduce repeated model loads by caching a template instance per modelName so new characters can be quickly created by cloning.
• Avoid reloading the same asset when multiple instances of the same character model are requested.
• Keep prior readiness and animation-preload behavior while enabling reuse of loaded assets.

Description

• Add flock.modelCache usage and flock.modelsBeingLoaded tracking to reuse or await an in-flight load before cloning a cached template.
• Introduce setTemplateFlags to mark cached template meshes as non-interactive and hidden, and create a finalizeMesh helper that centralizes per-instance setup such as setupMesh, material/color application, child mesh interactivity, animation preloading, and readiness announcement.
• On first load, clone a template from the loaded mesh and store it in the cache, and for subsequent requests clone from the cached template using the provided blockKey.
• Retain cleanup of readiness bookkeeping by deleting the modelReadyPromises entry after a short TTL with setTimeout and preserve abort/error handling paths.

Testing

• No automated tests were run for this change.
• Manual runtime validation was not recorded in automated output.

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Codex Task