The drag learns the stepper's trick — past the screen's edge, lane growth re-divides instead of stopping

The right-edge drag's growth was capped at the screen's visible frame,
because each snap tick grows the window; on a window near the screen edge
that left a lane stuck at a tick or two of headroom. Settled 2026-08-08
(03-board-ui.md § Lane, superseding the pathfinder's hard stop): at the
screen the window pins and each further tick re-divides the fixed strip
width across one more unit — siblings compress, the stepper's mechanism
arriving under the drag's fingers. The regimes meet with no pixel jump
(the re-divided standard at the fit IS the frozen standard, by the
exact-fill identity), shrinking mirrors the way back, the rubber band
moves to the strip's own capacity, and a window with no headroom at all —
full screen included — re-divides from the very first snap.

New pure arithmetic in LaneLayoutMath (pinnedStripWidth, resizeStandard,
resizeMaxUnits, resizeWindowDelta, snappedUnits over per-count slots);
LaneResizeSession splits the tick across the regimes and derives its
standard from the live count; the handle and BoardView hand the session
the strip's whole divide. 2709 unit tests green (+11).

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

CHANGELOG.md

@@ -1,5 +1,7 @@
 **August 2026**
 
+Dragging a lane's edge past the screen now keeps widening the lane by narrowing the others, instead of stopping — so wide lanes work even in full screen.
+
 Edit ▸ Undo, ⌘Z, and the toolbar's Undo and Redo buttons now work on board changes — they had been silently disabled.
 
 The board's symbol now appears in the title bar beside the board's name, in its chosen tint.