fix+docs v1.21.1: audit of v1.16-v1.21

- fix: Split snaps the click to the room's nearest wall instead of the grid, so
  it works on non-grid-aligned rooms (imported/legacy polygons); the pull is
  capped (~6 cells) so accidental mid-room clicks stay a miss; splitRoom() still
  rejects a bad cut. closestPointOnBoundary() + unit test.
- docs: README (en+ru) documents Merge/Split/ruler/per-space scale (were undocumented);
  TESTING.md merge/split rows + fresh self-run; new smokes smoke_merge_split,
  smoke_split_nonsnap (incl. the capped-pull case).
- 73 frontend + 12 backend tests green; module eval + both cards register.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "houseplan-card",
-  "version": "1.21.0",
+  "version": "1.21.1",
   "description": "Interactive house plan Lovelace card for Home Assistant",
   "license": "MIT",
   "type": "module",