fix: plan graph visuals — reading font, 1px lines, both-ends arrow color

Owner's three rendering refinements (plan-graph round, 2026-09-26):

1. Reading font: the diagram root sets font-family var(--font-prose) —
   node titles render in the reading font instead of the inherited mono
   UI font. Deliberately NOT the font-prose utility: @theme inline bakes
   the literal into utilities (the --font-title trap), which would freeze
   the graph at Source Sans 3 and ignore the Appearance reading-font
   override. The runtime var rides it (live-verified: Georgia override
   propagates, null resets to Source Sans 3).

2. Thinner lines: edges + box outlines 1.5 → 1 (the app's standard border
   weight). The arrowheads scale by the same 2/3 (HEAD_H 16 → 10.67; the
   width, the notch overhang, and the DOCK_GAP all derive from the head
   scale), so the head:shaft LaTeX proportion is preserved exactly.

3. Arrow color: white only when BOTH ends are white — an edge with an
   upcoming origin OR upcoming terminal stays gray (line + head).
   Supersedes round 6's target-only rule: arrows leaving a gray box no
   longer paint white into covered territory.

Live-verified on :3001 (native-format fixture: plan_init + answered
quizzes; popup): font default + override, stroke-width 1 on all rects and
edges, 5-edge graph with exactly one both-white edge white and both
upcoming→visited cross edges gray.

tests/plan-window.test.tsx

@@ -60,7 +60,7 @@ describe("PlanMarginalia render", () => {
 		assert.ok(!/viewBox="-?\d+(\.\d+)? -?\d+(\.\d+)? 360 180"/.test(html), "the windowed viewBox is retired");
 		// Meet-fit mode: the svg fills the band box (h-full w-full), letting
 		// preserveAspectRatio contain the diagram in the flank.
-		assert.ok(html.includes('class="block h-full w-full'), "fill mode classes");
+		assert.ok(html.includes('font-family:var(--font-prose)'), "fill mode classes");
 	});
 
 	it("glows the current node and dims the not-yet-visited (live statuses)", () => {