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/ARCHITECTURE.md

@@ -182,6 +182,17 @@ double click → properties dialog. In markup mode the "Opening" tool handles cl
 | `houseplan/plans/delete` | `name` | `{ok, removed}` / err `in_use` |
 | `houseplan/file/set` | `marker_id`, `filename`, `data` (b64) | `{ok,url,name}` (legacy, WS limit) |
 
+**The canvas is square, the image is not** (v1.48.0). A space used to carry an
+`aspect`, and coordinates were normalised against it — x by the width, y by the
+height. That made every geometric question depend on a per-space number for no
+benefit. Now the render space is `NORM_W × NORM_W` and a plan image is fitted
+inside it by its own ratio (`fitInSquare`, shared by both renderers), which is
+stored as `plan_aspect` so the layout does not jump before the file loads.
+Upgrading runs `geometry_migration.migrate_config` once: it pads the old box out
+to a square and re-expresses every coordinate against it — a uniform scale plus
+an offset in render units, so angles and proportions are exact — and scales
+`cell_cm` for tall plans, since the grid pitch is a fraction of the width.
+
 **User content is served inert** (HP-1454-01). An uploaded SVG is the only
 thing here that a browser will happily treat as a *document* rather than an
 image, and it would be a document of Home Assistant's own origin. Inside the