appearance-mask-eraser experiment

this branch keeps the classic brush and eraser engine intact while adding a separate Advanced brush path. the appearance experiment no longer swaps in a reduced copy of tools.rs, so classic nibs, eraser nib sync and eraser preview remain on the original tool path.

rollback

• switch back to main, or
• build q0editor with --no-default-features on this branch.

data and compatibility

• classic vector paint still commits as fill-only raw geometry.
• q1s v9 introduced sparse vector appearance metadata; q1s v10 adds frozen material-source and post-material clip paths; q1s v11 adds an exact 2x3 affine for the already-resolved appearance field.
• q0s v8 carries the original appearance masks, q0s v9 carries post-material fragments, and q0s v10 carries the resolved-field affine into q0player.
• older q1s/q0s versions remain readable and load with no synthetic appearance state.
• writing an older format version with appearance data fails instead of silently dropping it.
• v1 migration creates no appearance metadata.

appearance and eraser semantics

• classic brush paint is always plain vector fill and has no runtime material toggle. legacy Classic Glow settings migrate once into Advanced mode; Glow now belongs to Advanced materials.
• only the soft halo is rasterized from a gaussian alpha blur; the brush body remains the normal tessellated vector fill in q0editor.
• source fill alpha and halo alpha are resolved first. the erase mask is multiplied into that final material alpha afterwards.
• the mask eraser never subtracts from the source vector paths.
• the mask receives the exact classic eraser-nib coverage clipped to finite visible material support; halo radius is never added to the eraser footprint.
• regenerating the halo cannot paint back into an erased mask region.
• appearance metadata belongs to the vector asset inside the project, so undo/redo, save/reopen, nested q0rg rendering, transforms, export and q0player all see the same state.
• normal Select resolves the visible material surface: erased mask regions are not selectable, while finite glow support participates in hit-testing and bounds. Subselect still exposes only real vector anchors.
• ordinary assets without appearance metadata continue through the legacy vector renderer and classic geometry eraser.

this remains feature-gated while the visual/material model is being proven, but it is no longer runtime-only state.

• partial raw-fill split does not re-evaluate filters from each new contour. both fragments retain the pre-split material source and complementary post-filter clips; translating a fragment translates its material/erase/clip state together.
• a fragment's post-material clip is authoritative for rendering, hit-testing and selection bounds. hidden portions of the frozen source cannot start a drag, while a marquee containing only visible halo can still create and move a real fragment.
• raw move/scale/rotate/skew use one drag-start affine for the carrier vector and compose that same matrix into the resolved appearance field. the frozen material source is not skewed and blurred again: its already-resolved halo texture is rotated/scaled/sheared as a field, with clip and erase masks staying in that canonical field space.
• fragment rasterization is bounded around the post-material clip plus one finite filter radius instead of allocating a texture for the complete frozen source. editor halo quality is capped at 4 pixels per stage unit at extreme zoom so interactive cache rebuilds remain bounded.
• raw clipboard snapshots carry vector + appearance together. copy/paste/duplicate and Convert to q0rg therefore preserve frozen glow state under fresh asset ids instead of materialising clean vectors; halo-only marquee fragments keep an internal carrier plus their post-material clip.
• the selection hot path never rebuilds a soft-halo polygon buffer per frame: halo hit-testing uses distance to the finite source support, transform bounds use a conservative envelope, and selected glow uses the lightweight transform box instead of re-tessellating the filtered material.
• halo textures use transparent overscan and remain continuous underneath the real vector fill, avoiding raster/vector AA cracks. once a material source is frozen, affine edits reuse that canonical halo texture; changing the frozen source/clip/erase content invalidates it.