feat: add compact multi-page atlas packing

[jusias]

Summary

• add sprite-gen compact-atlas, a deterministic MaxRects BSSF post-compose packer
• trim transparent frame bounds without resampling and emit equal-size Texture2DArray-compatible pages
• preserve animation clips on one page by default; optionally allow cross-page clips only when the configured empty-space threshold is exceeded
• write per-frame page, atlas rect, logical sourceX/sourceY, and a machine-readable packing report
• let direction-prefixed locomotion states such as east_run and south_move inherit the run/move prompt requirements
• expose the command through the package CLI and compatibility script, with SKILL/README documentation

Compatibility

The source atlas and manifest remain unchanged. Outputs use new page PNGs plus manifest.compact.json, so existing compose and curation workflows continue to work. The retired motion-phase drawing feature is not restored.

Validation

• compact-atlas tests: 3 passed
• CLI and package exposure tests: 25 passed
• direction-prefixed state inheritance tests: 2 passed

[jusias]

Closing this upstream proposal because the extension will be maintained in a personal fork for project-specific use.