feat: reset presentation state on subject replacement

[han4wluc]

Summary

• treat an explicitly authored presentation subject as a fresh instance for backgrounds, character items, and visual items
• reset every omitted instance field, including appearance, placement, layer, backing color, resource playback overrides, and animations, even when the same subject is reauthored
• preserve the current instance for omitted channels and subject-less continuation patches, including partial visual-text patches
• mark history-dependent subject-less continuation patches as compatibility-only behavior planned for removal in a future version
• document the complete-action authoring contract and migration impact in the public guide and action schema
• ship the behavior as feature version 1.46.0 and sync the branch with main at v1.45.2

Subject contract

• Background: resourceId
• Character item: non-empty sprites
• Visual item: resourceId, complete layout, or complete text containing content and textStyleId

A subject-bearing action is complete. Fields omitted from it use normal defaults. To patch the current instance, omit the subject; this patch form remains supported in v1.46.0 for compatibility but new content should not rely on it.

Compatibility

This intentionally changes older content that reauthored a subject while relying on inherited instance fields. Conforming authoring tools already emit complete subject-bearing actions, so practical risk is expected to be low. The migration notes identify the affected behavior present in recent versions.

Validation

• bun run test: 84 files and 2,068 tests passed
• bun run build
• bun run lint
• bun run check:test-markers: 158 files scanned
• Prettier check for every changed source, test, schema, documentation, and VT spec
• isolated background, character, and visual VT capture and report: 3/3 images passed per target, zero mismatches
• live Chromium reproduction: continuation patch retained placement and blur; identical resource reassertion reset placement and appearance
• full Docker VT capture: 196/198 cases captured; two unrelated audio-channel assertions failed (audio-effects-handoff and audio-effects-initial-entry)