Merge lh-graph refinement wave (feat/edge-glyph @ 66e6212) into main

Edge glyph (⟹ follows arrow) WITHDRAWN by owner after two live-rejected
variants — CanvasEdge.type plumbing + ArrowGeom retained, differentiation
approach still open (docs/vision-edge-glyph.md decision record).

Keepers: regions.enabled "Background colors" compute off-switch (skips
splat/blur/readback/blit), clusterLabelSize (default 14, topmost draw
layer, screen-px) with Cluster labels toggle + size slider in the Regions
section, dependency graying across the settings panels, physics panel
reorder (general forces first, Depth anchor + conditionals last).

Merged cleanly over master's coverage-chrome changes in graph-view.tsx
(their gated fragment + the ungated GraphCoverageControl coexist); data/
scratch not carried (untracked, master's real render-settings.json picked
up clusterLabelSize=14 via sanitizer fallback). Gates 488/488; live-
verified: chrome intact, Background colors graying works, physics order,
coverage panel opens (11 sessions).

docs/vision-edge-glyph.md

@@ -1,8 +1,18 @@
 # Vision — The edge glyph: follows vs supports
 
-Ideation settled 2026-09-14. Not yet planned, not yet assigned. Companion: `vision.md` § "Edges" (the relation types) and § "Rendering".
+Ideation settled 2026-09-14. **WITHDRAWN 2026-09-15**: both shipped variants of
+the double-shaft glyph (tails converging on a shared head; two complete arrows
+offset ±half-gap) were rejected by the owner live — the first "two arrows
+feeding one point", the second "two arrows next to each other, not coherent".
+Dashed was already rejected 2026-09-14 (dash patterns crawl under the constant
+screen-space stroke width). **The relation still needs a differentiator —
+approach open.** The constraints below that are NOT glyph-specific still stand.
+Companion: `vision.md` § "Edges" (the relation types) and § "Rendering".
 
-One line: the two relation types become visually distinct — **`follows` renders as a double-shaft arrow** (two parallel tails side by side, the implication glyph ⟹), **`supports` stays a single line**. Arrows keep their existing direction: prerequisite → dependent.
+One line (withdrawn): the two relation types become visually distinct —
+**`follows` renders as a double-shaft arrow** (two parallel tails side by
+side, the implication glyph ⟹), **`supports` stays a single line**. Arrows
+keep their existing direction: prerequisite → dependent.
 
 ---
 
@@ -21,7 +31,8 @@ Dash patterns break under our **constant screen-space stroke width**: the dash a
 
 The renderer is the custom 2D canvas; edges draw in the edge layer with shafts + arrowheads from `arrowGeometry` (sigma-style head, prerequisite → dependent). For `follows`:
 
-- Both shafts offset along the edge normal by a **fixed screen-space gap**, converging on a **shared single arrowhead**.
+- Both shafts offset along the edge normal by a **fixed screen-space gap**.
+- **REVISED 2026-09-15 (owner rejected the first cut):** the converged variant — two tails merging into one shared arrowhead — read as two arrows feeding a single point, not as parallel strokes. The glyph is two **complete regular arrows** (own shaft + own head each) translated ±half-gap along the normal: parallel shafts AND parallel heads, nothing converges.
 - The gap is proportional to the constant stroke width — a constant visual ratio at every zoom level.
 - **No LOD collapse (withdrawn 2026-09-14).** The earlier idea of collapsing to a single shaft below a stroke-width threshold assumed width shrinks with zoom — it doesn't; stroke width is constant on screen, so the double shaft renders identically at every zoom level. Simpler than proposed: nothing to gate.