feat v1.21.0: merge and split rooms

- Merge: click a room, then a neighbour. Adjacency is decided by the RESULT, not a
  heuristic: mergeRooms unions the outlines and accepts only when they collapse into one
  hole-free outline (corner touch / apart / hole => refused). A dialog picks the surviving
  name+area; the kept room keeps its id so its label and devices stay put.
- Split: click the room, then two wall points; the chord cuts it, live ruler on the cut.
  The bigger part stays the room (name/area/devices), the smaller opens the new-room
  dialog. The cut is applied only on confirm — Cancel leaves the room whole.
- Boolean geometry via polyclip-ts (proper ESM + native types; polygon-clipping ships
  named types but a default-only ESM build, breaking either tsc or the runtime).
  Verified on the real plan, where neighbouring walls overlap collinearly rather than
  match exactly — the case a hand-rolled union gets wrong. Bundle 151->202 KB.
+5 tests (72). Verified live: merge of Сауна+с/у -> 4-vertex outline, non-adjacent
refused, split preserves area (26667 -> 13333+13333), cancel leaves the room whole.

docs/CHANGELOG.md

@@ -1,5 +1,24 @@
 # Changelog
 
+## v1.21.0 — 2026-07-16 (merge and split rooms)
+- **Merge** (toolbar "Merge"): click a room, then a neighbour. Only rooms that **share a wall**
+  can merge — and that is decided by the result rather than a heuristic: `mergeRooms` unions the
+  outlines and accepts the pair only when they collapse into ONE hole-free outline. A corner
+  touch, rooms apart, or a union enclosing a hole are refused. A dialog picks which name and
+  area survive; the kept room keeps its id, so its label position and devices stay put. The
+  dialog warns that the other area is released.
+- **Split** (toolbar "Split"): click the room, then two points on its walls — the chord cuts it
+  in two, with the live ruler on the cut. **The bigger part stays the room it was** (name, area,
+  devices); the smaller becomes a new room and its dialog asks for name/area. Cancelling the
+  dialog leaves the room whole — the cut is applied only on confirm. Cuts that do not run
+  wall-to-wall inside the room (ends off the wall, chord leaving a concave room, a chord along a
+  wall) are refused.
+- Boolean geometry via **polyclip-ts** (proper ESM + native types; `polygon-clipping` ships named
+  types but a default-only ESM build, which breaks either tsc or the runtime). Verified against
+  the real plan, where neighbouring walls overlap collinearly instead of matching exactly —
+  the case a hand-rolled union gets wrong. Bundle: 151 KB → 202 KB.
+- New pure helpers: `polygonArea`, `mergeRooms`, `splitRoom`. (+5 tests: 67 → 72.)
+
 ## v1.20.0 — 2026-07-16 (rooms may not overlap)
 - **A click strictly inside an existing room is refused** while drawing (toast names the room).
   Being *on* a wall stays legal — neighbouring rooms share walls, and real walls overlap

docs/STATUS.md

@@ -13,14 +13,14 @@
 
 | Item | State |
 |---|---|
-| Version | **v1.20.0** everywhere (manifest, const.py, package.json, CARD_VERSION) |
+| Version | **v1.21.0** everywhere (manifest, const.py, package.json, CARD_VERSION) |
 | GitHub | https://github.com/Matysh/houseplan-card — branch `main`, releases v1.9.3…v1.11.2 |
 | CI | `.github/workflows/validate.yml` (hacs + hassfest + frontend + backend) — **fully green** since v1.11.1; `release.yml` auto-attaches the card bundle (needs `permissions: contents: write`, fixed) |
 | HACS | Works as custom repository (id 1290210112 on the home instance). **Inclusion PR: https://github.com/hacs/default/pull/9004** (queue ≈2 months as of 2026-07). Lesson: #8995 was auto-closed by hacs-bot — the PR body MUST be their exact template with every checkbox ticked and all 3 links (release, HACS action run, hassfest run); a custom body gets closed without discussion |
 | Brands | Ships **inside the integration**: `custom_components/houseplan/brand/{icon,icon@2x,logo,logo@2x}.png` (HA ≥2026.3 local-brands mechanism). home-assistant/brands PR #10700 was auto-closed — that repo no longer accepts custom integrations |
 | Home instance | ha.jbstudio.pro (SSH port 323, key `ha_jb`), deployed v1.11.2, installed *via HACS* (custom repo) — updates flow through HACS now |
 | Localization | UI en/ru (`src/i18n.ts`), auto by `hass.locale` + `language` card option; codebase and docs are English-first (`README.ru.md` is the Russian copy) |
-| Tests | 67 frontend (node:test, incl. a 12-test buildDevices suite on a fake hass) + 10 pure backend (anywhere) + 12 HA-harness backend (CI only, py3.13; skipped locally — sandbox has py3.10) |
+| Tests | 72 frontend (node:test, incl. a 12-test buildDevices suite on a fake hass) + 10 pure backend (anywhere) + 12 HA-harness backend (CI only, py3.13; skipped locally — sandbox has py3.10) |
 
 ## Recent milestones (details in CHANGELOG.md)
 

docs/TESTING.md

@@ -74,6 +74,11 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
 - [ ] Rooms never overlap (v1.20.0): a click strictly inside an existing room is refused with a
       toast; a click ON a shared wall (including mid-span of a longer neighbour wall) still works
 - [ ] Closing an outline drawn AROUND an existing room is refused; the outline stays open
+- [ ] Merge (v1.21.0): two rooms sharing a wall merge into one; the dialog picks the surviving
+      name/area; rooms touching only at a corner or apart are refused with a toast
+- [ ] Split (v1.21.0): click a room, then two points on its walls — the bigger part keeps the
+      name/area/devices, the smaller opens the new-room dialog; Cancel leaves the room whole
+- [ ] Split: a cut with an end off the wall, or along a wall, is refused with a toast
 - [ ] Esc / Ctrl+Z removes the last dot (and its line); Reset clears the path
 - [ ] Closing the contour (click the first dot, ≥4 points) opens the room dialog
 - [ ] Room dialog: area list shows only unassigned areas; picking an area prefills the name