v1.48.0: the canvas is always square, the plan is centred inside it

A space carried an aspect ratio, and coordinates were normalised against it: x
by the width, y by the height. Every geometric question therefore depended on a
per-space number, and picking a canvas orientation was a decision the user had
no reason to make. The render space is now NORM_W x NORM_W and a plan image is
fitted into it by its OWN ratio, centred — wide plans get margins above and
below, tall ones at the sides.

Migration (geometry_migration.py, pure and unit-tested) runs once at setup under
the write lock. Nothing about a drawing changes: the old box is padded out to a
square and every coordinate re-expressed against it — rooms as rects and
polygons, openings and their lengths, decor, view_box, and the marker positions
in the separate layout store. In render units it is a uniform scale plus an
offset, so angles and proportions are exact. cell_cm is scaled for tall plans,
because the grid pitch is a fraction of the width: without it a wall would
measure less than it does.

 is now dropped by the schema rather than accepted — a stale tab sending
it would be sending coordinates from the old normalisation too, and honouring
the field would not make them right.

The demo fixture was migrated with the same transform, so the smokes exercise
the new geometry rather than a square-native fake; six of them needed their
render-space helpers updated and one its click coordinates.

Not released — dev only, per the owner's instruction.

docs/TESTING.md

@@ -239,6 +239,15 @@ 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]
+- [ ] Square canvas migration (v1.48.0): after the upgrade every existing plan
+      looks exactly as before, just with margins where the canvas was extended.
+      Measure a wall in the plan editor — the length in cm is unchanged. Marker
+      positions, doors, decor and the saved zoom are all where they were
+      [auto: unit: test_a_wide_plan_gains_margins_above_and_below and neighbours,
+      test_migration_preserves_real_lengths_and_shapes]
+- [ ] A plan image is centred (v1.48.0): a wide image sits in the middle with
+      empty bands above and below, a tall one with bands at the sides, and it is
+      never stretched [auto: unit: fitInSquare + smoke_space_settings]
 - [ ] Re-attaching a detached plan (v1.47.0): detach a plan, save, RELOAD THE
       PAGE, open space settings → "Already uploaded" → the image is listed with
       its size and no "in use" note → attach it → it renders. The one a space