perf(stairs): draw all treads of a stair as one path (#740)

A floor with stairs pays for them on every switch to it: the stair layer
is emptied on other floors, so Lit recreates every symbol on each return
and the browser lays out and paints it again. With 250 stairs (the
large-house fixture, the per-floor limit) that was 2,875 SVG elements and
about 40 ms per entry locally; each stair carried 3-7 separate tread lines
with four bound coordinates each.

The treads of one stair are now a single <path class="hp-stair-tread">
with one `M a L b` subpath per tread, in geometry order and with the
numbers the lines carried. Treads of one stair never overlap (straight:
parallel, >= 20 cm apart; spiral: inner ends >= 6.7 cm apart at the
3.6 cm stroke), so the path paints the same pixels at any opacity. The
outline points and the tread data are built once per cached geometry
object (cachedStairMarkup, weak keys), not on every render. The View and
plan-editor layers share the strings; outline, hit polygon, trapezoid,
arrow, attributes and handlers are unchanged. Floor 1 of the fixture
drops from 4,210 to 2,960 elements.

Witnesses: the unit test for the path data and its cache, and the
smoke_stairs markup checks in View and in the plan editor, are red on
dev. The stairs-view-tread-lines mutant restores the View lines.

Issue: #740
User-Visible: no
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018qZfe7YS4rqEMKoVeS3GKd

docs/STAIRS.md

@@ -145,7 +145,10 @@ links are cleared; a one-space transfer cannot invent an external target.
 ## Implementation boundary
 
 - `src/stairs.ts` owns validation, drawing geometry and area-subtraction
-  primitives.
+  primitives. The View and plan-editor symbols draw all treads of a stair as
+  one `<path class="hp-stair-tread">` (one `M a L b` subpath per tread, #740);
+  its `d` and the outline points are built once per cached geometry object
+  (`cachedStairMarkup`), not on every render.
 - `src/stairs-view.ts` is the eager read-only boundary for symbols, guarded
   navigation and touch/pointer gesture suppression.
 - `src/stairs-editor-model.ts` keeps the eager-safe helpers the View runtime