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