Render pending placeholders for prompt-only media assets

[dazzatronus]

Schemas 1.13 lets media assets (image/video/audio) carry a prompt instead of
a src, with generation filling src in place. Studio rendered these as nothing.

• Prompt-only assets get the AI pending treatment: canvas overlay with numbered
  prompt label, AI icon + label on the timeline. Per-asset, so plain src media is
  unaffected. A pending video with a seed previews the seed image.
• Legacy generative types and unified assets of one kind share a numbering sequence.
• When generation fills src, the reconciler recreates the clip as a normal media
  player. Realised assets keep their AI timeline label.
• ClipUnresolved fires for any pending asset and now also emits assetType
"image" | "video" | "audio" — host apps switching on that field take note.

No new rendering code: two pure guards rekey the legacy placeholder system, the
factory routes on pending state, the reconciler recreates on pending flips.

Verify: npm run dev loads a template with every state. 14 new unit tests; suite,
typecheck, lint green.

Includes the schemas 1.13 bump and no-src load guards from #123 (superseded). Bundles @shotstack/schemas 1.13.1 (supersedes the dependabot bump) and @shotstack/shotstack-canvas 2.10.2.
Follow-ups: prompt-editing toolbar, Generate action, export gating for unrealised assets.