User and imported .abr brush presets now survive a restart. The new
photocraft_engine::preset_store keeps one JSON file per preset group plus
content-addressed, deflated tip bitmaps (8-bit when the tip came from 8-bit
data) in <config dir>/Presets, and an index with group order and deleted
built-ins. Built-ins are never written; writes are atomic and bounded;
corrupt or oversized files are skipped with a warning and left on disk.
The desktop app loads the store on a background thread and attaches it when
it arrives; sessions without a store (CLI, MCP, web, tests) stay hermetic.
Brush preset mutations bump ToolState::presets_rev and the store syncs after
the command, rewriting only groups whose content changed.
Fixes#78
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
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