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 <[email protected]>

src/entities/dimension.rs

@@ -3882,6 +3882,7 @@ fn tessellate_dimension_inner(
             let (ext1, ext2) = split_ext_lines(&geom.ext_lines);
             if !ext1.is_empty() {
                 wires.push(WireModel {
+                    bg_adapt: None,
                     point_marker: None,
                     taper_widths: Vec::new(),
                     pattern_stations: Vec::new(),
@@ -3917,6 +3918,7 @@ fn tessellate_dimension_inner(
             }
             if !ext2.is_empty() {
                 wires.push(WireModel {
+                    bg_adapt: None,
                     point_marker: None,
                     taper_widths: Vec::new(),
                     pattern_stations: Vec::new(),
@@ -3952,6 +3954,7 @@ fn tessellate_dimension_inner(
             }
         } else {
             wires.push(WireModel {
+                bg_adapt: None,
                 point_marker: None,
                 taper_widths: Vec::new(),
                 pattern_stations: Vec::new(),
@@ -3988,6 +3991,7 @@ fn tessellate_dimension_inner(
     }
 
     wires.push(WireModel {
+        bg_adapt: None,
         point_marker: None,
         taper_widths: Vec::new(),
         pattern_stations: Vec::new(),
@@ -4032,6 +4036,7 @@ fn tessellate_dimension_inner(
         let mut points = Vec::new();
         add_polyline(&mut points, &symbol);
         wires.push(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),
@@ -4080,6 +4085,7 @@ fn tessellate_dimension_inner(
                     aci_to_rgba(&c)
                 };
                 wires.push(WireModel {
+                    bg_adapt: None,
                     point_marker: None,
                     taper_widths: Vec::new(),
                     pattern_stations: Vec::new(),
@@ -4130,6 +4136,7 @@ fn tessellate_dimension_inner(
             let p3 = rect[2];
             let p4 = rect[5];
             wires.push(WireModel {
+                bg_adapt: None,
                 point_marker: None,
                 taper_widths: Vec::new(),
                 pattern_stations: Vec::new(),

src/entities/leader.rs

@@ -672,6 +672,7 @@ impl LeaderTess for Leader {
 
         if verts.len() < 2 {
             return WireModel {
+                bg_adapt: None,
                 point_marker: None,
                 taper_widths: Vec::new(),
                 pattern_stations: Vec::new(),
@@ -852,6 +853,7 @@ impl LeaderTess for Leader {
             crate::scene::convert::tessellate::points_to_ds(fill_tris);
 
         WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),

src/entities/multileader.rs

@@ -2068,6 +2068,7 @@ impl MultiLeaderTess for MultiLeader {
         // WireModels so the renderer respects per-piece coloring.
         let mut wires: Vec<WireModel> = Vec::new();
         wires.push(WireModel {
+            bg_adapt: None,
             point_marker: None,
             taper_widths: Vec::new(),
             pattern_stations: Vec::new(),
@@ -2367,6 +2368,7 @@ impl MultiLeaderTess for MultiLeader {
                         xy = xy.max(p[1] as f64);
                     }
                     wires.push(WireModel {
+                        bg_adapt: None,
                         point_marker: None,
                         taper_widths: Vec::new(),
                         pattern_stations: Vec::new(),
@@ -2441,6 +2443,7 @@ impl MultiLeaderTess for MultiLeader {
                         pts.push([bx, by, z]);
                     }
                     wires.push(WireModel {
+                        bg_adapt: None,
                         point_marker: None,
                         taper_widths: Vec::new(),
                         pattern_stations: Vec::new(),
@@ -2530,6 +2533,7 @@ impl MultiLeaderTess for MultiLeader {
                         wcs_corners[3],
                     ];
                     wires.push(WireModel {
+                        bg_adapt: None,
                         point_marker: None,
                         taper_widths: Vec::new(),
                         pattern_stations: Vec::new(),
@@ -2574,6 +2578,7 @@ impl MultiLeaderTess for MultiLeader {
                         wcs_corners[0],
                     ];
                     wires.push(WireModel {
+                        bg_adapt: None,
                         point_marker: None,
                         taper_widths: Vec::new(),
                         pattern_stations: Vec::new(),

src/entities/table.rs

@@ -1990,6 +1990,7 @@ pub fn tessellate_table(
     let mk =
         |color: [f32; 4], points: Vec<[f32; 3]>, fill_tris: Vec<[f32; 3]>, lw: f32| -> WireModel {
             WireModel {
+                bg_adapt: None,
                 point_marker: None,
                 taper_widths: Vec::new(),
                 pattern_stations: Vec::new(),