Typography round 4 (owner): the snap to the ladder — prose 16.5→16, xxs rung merged into xs

The system: the ladder is the truth, everything snaps to it.

- Prose snaps to the ladder's 16px rung (--lh-prose-size 16.5→16; the
  half-pixel was a reskin-era hand-tune with no visible payoff and a
  system cost). lh-prose, lh-input, and the session titles all read the
  one variable — one reading size, now a ladder citizen. @theme inline
  inlines literals without emitting runtime vars, so the variable can't
  var()-link to --text-lg: documented as the one deliberate duplication
  (change one, change both).
- The 10px rung merges into 11px: one pixel apart was a distinction with
  no visible difference, and 10px sits at the mono legibility floor —
  quiet labels (Sessions, Background agents, dock hints, model-picker
  footers) get their quiet from color + tracking, not a notch of size.
  All 13 text-xxs uses → text-xs; --text-xxs token removed.
- Derived constant follows: TEXT_CAP_SCALE 16.5/12.5 → 16/12.5 (1.32 →
  1.28; test pin 232.32px → 225.28px) — the plan-graph text cap now
  tracks the snapped prose size.

Live: title 16px, Sessions/Background-agents labels 11px, composer 16px
with the 20px lh-input line box. Gates: typecheck, lint, 469/469.

Lane note: components/graph-context-marker.tsx (graph-stream file) had a
mechanical text-xxs → text-xs swap — required: the token removal
otherwise leaves a dead class in their file.

tests/render-plan-block.test.tsx

@@ -130,9 +130,10 @@ describe("plan block render", () => {
 		assert.ok(!/data-testid="plan-block-full"[^>]*border/.test(html), "no wrapping chrome");
 		// Fills the column UP TO the chat-font cap: the svg is w-full +
 		// mx-auto with maxWidth = extent × TEXT_CAP_SCALE, so node text never
-		// renders past the 16.5px prose size (owner, plan-graph round 1).
+		// renders past the 16px prose size (owner, plan-graph round 1; the
+		// snap 16.5→16 landed in typography round 4).
 		assert.match(html, /<svg[^>]*class="block h-auto w-full mx-auto"/);
-		assert.match(html, /max-width:232\.32px/); // (160+16) × 1.32
+		assert.match(html, /max-width:225\.28px/); // (160+16) × 1.28
 		// Two stacked nodes: 2*NODE_H + GAP_Y = 136 extent + GRAPH_PAD on
 		// every side (strokes/glow never clip at the edges).
 		assert.match(html, /viewBox="-8 -8 176 152"/);