perf: let one tessellation serve every background

Switching to a Layout re-tessellated the whole model — `memo_miss=186455`,
2034 ms — to arrive at geometry identical to what was already in the memo.
The only thing that differed was colour.

`local_wires_for` already knows this: it stores the raw colour and drops
its background argument with `let _ = bg_color;`, noting that
`Batches::finalize` applies `adapt_to_bg` last, "so the cache no longer
has to rebuild on BACKGROUND / layout-switch". The tessellation memo's
guard had not been told, and still mixed the background in, so every
entry was discarded on a switch.

`finalize` now records what its colour resolution consumed — the raw
colour, the raw glyph colours, and the three flags it branches on — as
`WireModel::bg_adapt`, and `resolve_colors_for_bg` replays it for another
background. The guard drops the background, so a memo entry serves any
layout.

The resolution recomputes from the recorded raw colour rather than from
the resolved one, which makes it idempotent and therefore safe over a set
that mixes memo hits with freshly tessellated misses. Re-adapting the
resolved colour would not be safe: `adapt_to_bg` snaps a near-white colour
to pure black on a light background, and pure black comes back pure
*white* on a dark one, losing the tint. A test pins that asymmetry so the
raw colour is not optimised away later.

The contract is tested against the real `finalize` rather than a copy of
its logic: for every background pair, building under one and resolving for
the other lands exactly where building under the other would have.

`bg_adapt` is `None` for wires built anywhere else, and the resolution
skips them, so nothing outside the block cache changes colour. It is also
`None` when the resolution would be a no-op — a preserved colour with no
canvas fill looks the same under every background — which keeps the common
case at no cost.

Ordering is unchanged: the pass runs inside the builder, before
`append_scene_markers` and `apply_refedit_fade`, both of which have their
own separate background dependency.

`wires-build` gains `colors=` so the pass has a number of its own.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/modules/draw/draw/attdef.rs

@@ -199,6 +199,7 @@ impl CadCommand for AttdefCommand {
             pt + self.plane.y * d,
         ];
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/draw/mline.rs

@@ -180,6 +180,7 @@ impl MlineCommand {
         let (fill_tris, fill_tris_low) =
             crate::scene::convert::tessellate::points_to_ds(fill_tris);
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/draw/raster_image.rs

@@ -123,6 +123,7 @@ impl CadCommand for ImageCommand {
         let [p0, p1, p2, p3] = corners;
 
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/draw/ray.rs

@@ -85,6 +85,7 @@ impl CadCommand for RayCommand {
         let dir = (pt - base).normalize_or_zero();
         let far = base + dir * DISPLAY_EXTENT;
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),
@@ -189,6 +190,7 @@ impl CadCommand for XLineCommand {
         let far_pos = base + dir * DISPLAY_EXTENT;
         let far_neg = base - dir * DISPLAY_EXTENT;
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/inquiry/area.rs

@@ -365,6 +365,7 @@ impl CadCommand for AreaCommand {
         points.push(to_render(point));
         points.push(to_render(self.points[0]));
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/inquiry/dist.rs

@@ -86,6 +86,7 @@ impl CadCommand for DistCommand {
         let p1 = self.first?;
         // The preview wire is purely visual, so f32 vertices are fine here.
         Some(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/inquiry/measuregeom.rs

@@ -70,6 +70,7 @@ impl MeasureGeomCommand {
     /// Build a cyan preview wire connecting the picked points and the cursor.
     fn preview_wire(name: &str, pts: Vec<[f32; 3]>) -> WireModel {
         WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/modules/draw/modify/trim.rs

@@ -3746,6 +3746,7 @@ fn collect_runs(pts: &[[f64; 2]], take: &dyn Fn([f64; 2]) -> bool, out: &mut Vec
 /// Build a preview wire from a NaN-break point list.
 fn preview_wire(points: Vec<[f32; 3]>, color: [f32; 4], name: &str) -> WireModel {
     WireModel {
+        bg_adapt: None,
         point_marker: None,
         taper_widths: Vec::new(),
         pattern_stations: Vec::new(),