v1.50.4: one model builder for both cards (HP-1503-01)

The full card's _buildModel() was a hand-copied twin of spaceModels(), and
the twin missed the legacy-store fallbacks v1.50.3 gave the shared builder —
the same broken store rendered recovered in the static card and as
viewBox='0 0 0 0' with negative-width rects in the main one. The divergence
of the duplicates IS the bug, so the duplicate is gone: the full card calls
spaceModels() and only swaps the raw plan url back in (its signing flow must
not bake a signed url into a memoized model — 2026-07-27).

New smoke_legacy_geometry runs the audit's exact vector (zero viewport +
negative rect) through both models and both DOM trees and asserts parity:
full-canvas fallback, normalised rectangle, no negative SVG attributes.
Inventory: 140 / 51 / 43 / 65.

docs/TESTING.md

@@ -239,6 +239,9 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
       on the plan after a reload. Same for each tap action and each fill mode
       [auto: backend test_every_display_mode_the_editor_offers_is_accepted and
       neighbours, test_a_marker_showing_its_value_can_be_saved]
+- [ ] Legacy geometry parity (v1.50.4, HP-1503-01): a store with a zero
+      viewport and a negative rect renders identically sane in BOTH cards —
+      full canvas fallback, normalised rectangle [auto: smoke_legacy_geometry]
 - [ ] Sizes are positive (v1.50.3, HP-1502-01): view_box or room w/h of zero
       or below is refused; a store that already holds one opens on the full
       canvas, not a blank screen [auto: test_sizes_are_not_coordinates + unit