feat(backend): LED strips are stored, normalised and transferred (#780)

Stage 1 of #780 — the data model. A space carries an optional
`led_strips: [{id, points, marker, active?}]` (custom_components/houseplan/
led_strips.py, pure, strict mypy).

- Type schema inside SPACE_SCHEMA: 2–50 finite numeric points (no strings,
  booleans, NaN or off-canvas values), ≤50 strips per space including hidden
  shapes, strict boolean `active`, marker = non-empty string or null.
- A config-level step after coordinate canonicalisation judges the shape
  (two distinct points, non-zero length, a closed strip needs three distinct
  vertices, a hidden shape needs a marker, unique ids per space) and the links
  in the order the spec fixes: duplicates are rejected before any
  normalisation (two links to the same missing id still conflict); a link to
  a marker that is not live becomes an unbound strip (marker null, active
  true, id and points kept), so a client that does not know strips can delete
  a bound marker without its save failing; a live marker with an empty space
  adopts the strip's space; a non-empty foreign space rejects the write.
- config/set answers with `led_strips: {unbound, space_adopted}` when the
  write was normalised, so a new client re-reads; old clients ignore it.
- Space import remaps links through the marker id map; a skipped or
  virtualised duplicate leaves the strip unbound; a coinciding old id never
  binds. Plan-only export keeps geometry and nulls every link. Import details
  report `unbound_led_strips`, computed by the server, never read from the file.
- Coordinates get JSON-noise cleanup only, like stairs (face contacts are
  off-lattice), in both canonicalisers with a shared fixture case.
- Support package: counters only (total/unbound/hidden), no coordinates or ids.

Tests: tests_backend/test_led_strips.py (41, pure), test_ha_import_export
(6 cases: full round trip with a hidden shape, orphan count, remap against a
coinciding id, skip and virtual duplicates, plan-only), test_ha_websocket
(old client deletes a bound marker → save stands, counters, foreign space
rejects without a new revision). Full backend with the HA harness: 969 passed.
Mutating the shape check, the duplicate check, the orphan normalisation or the
space adoption each turns the pure suite red.

Issue: #780
User-Visible: no

src/coordinate-canonicalization.ts

@@ -96,6 +96,16 @@ function isDecorBoxKind(value: unknown): value is DecorBoxKind {
     && (DECOR_BOX_KINDS as readonly string[]).includes(value);
 }
 
+function scalarPoints(value: unknown): void {
+  if (!Array.isArray(value)) return;
+  for (const point of value) {
+    if (!Array.isArray(point)) continue;
+    for (let index = 0; index < Math.min(2, point.length); index++) {
+      point[index] = canonicalizeNumber(point[index]);
+    }
+  }
+}
+
 function scalarFields(item: JsonRecord, names: readonly string[]): void {
   for (const name of names) {
     if (Object.prototype.hasOwnProperty.call(item, name)) {
@@ -382,6 +392,10 @@ export function canonicalizeConfigGeometryInPlace<T>(config: T): T {
       latticePoint(span.a);
       latticePoint(span.b);
     }
+
+    // #780: LED strips snap to physical wall faces like stairs: JSON noise
+    // only, a face contact never moves onto a lattice node.
+    for (const strip of records(space.led_strips)) scalarPoints(strip.points);
   }
 
   for (const marker of records(root.markers)) scalarFields(marker, ['angle']);