Sprite pipeline: plate colour missing from key/alpha fingerprints; lexical frame sort; null tolerance in from_dict #46

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Follow-ups from the PR #45 review (non-blocking suggestions). Sprite tab shipped in v0.47.0/v0.47.1.

1. core/sprite/pipeline.py: project.plate_color is absent from the key and alpha stage fingerprints, yet auto_key_color falls back to it when the border is not uniform and alpha_runner uses it when no key report exists. Changing the plate colour and rerunning serves the cached frames. Add the plate colour to both fingerprints when key_color is auto.
2. core/sprite/extract.py: sorted(temp.glob("*.png")) is lexical, so 10000.png sorts before 2000.png in exact_n mode on long clips. Sort by int(p.stem).
3. core/sprite/project.py from_dict: an explicit null for actions, frames, cost_ledger or cell_size raises a bare TypeError; load() only converts JSON errors into a user message. Tolerate null and report other shape errors as a user-facing ValueError. Also: stage_fingerprints is mutated by the worker while to_dict iterates it under the save lock (the code comment acknowledges this); snapshot it first.

Source: review comment on #45