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.

components/plan-block.tsx

@@ -192,8 +192,9 @@ export const GLOW_BLUR = 5;
 export const GLOW_REACH = GLOW_MARGIN + 3 * GLOW_BLUR;
 /** Node titles render at text-xs (12.5px) in svg units. The in-chat block
  *  scales up to fill the column but NEVER past the chat prose size
- *  (--lh-prose-size, 16.5px) — the width cap that makes that hold. */
-export const TEXT_CAP_SCALE = 16.5 / 12.5;
+ *  (--lh-prose-size, 16px — the ladder's reading rung) — the width cap
+ *  that makes that hold. */
+export const TEXT_CAP_SCALE = 16 / 12.5;
 
 // ─── The owner's arrowhead (plan-graph round 5) ──────────────────────────────
 
@@ -808,7 +809,7 @@ export function PlanPopup({ messages, onClose }: { messages: readonly PopupMessa
 							/>
 						</div>
 						<div className="pointer-events-none absolute inset-x-0 bottom-0 flex items-end justify-between p-2">
-							<span className="text-xxs text-muted-foreground/60">Drag to pan · scroll to zoom</span>
+							<span className="text-xs text-muted-foreground/60">Drag to pan · scroll to zoom</span>
 							<div className="flex gap-1">
 								<button
 									type="button"