[puppeteer] Report layers with keyframes but no runtime proxy

[S1mplector]

Module: Puppeteer

Goal:
Detect and explain cases where a layer has keyframes in Puppeteer but cannot resolve to a runtime layer proxy.

Acceptance criteria:

• Diagnostics panel lists unresolved layer ids with keyframe counts.
• Export confirmation warns if unresolved layers will not affect runtime playback.
• Suggestions include checking expression layer ids and rig persistence.
• Existing valid projects show no false-positive warnings.

Notes:
This should make issues like missing orbit/arm proxy mapping easier to debug.