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.

package-lock.json

@@ -1,15 +1,16 @@
 {
   "name": "houseplan-card",
-  "version": "1.15.0",
+  "version": "1.20.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "houseplan-card",
-      "version": "1.15.0",
+      "version": "1.20.0",
       "license": "MIT",
       "dependencies": {
-        "lit": "^3.1.3"
+        "lit": "^3.1.3",
+        "polyclip-ts": "^0.16.8"
       },
       "devDependencies": {
         "@mdi/js": "^7.4.47",
@@ -597,6 +598,15 @@
         "node": ">=0.4.0"
       }
     },
+    "node_modules/bignumber.js": {
+      "version": "9.3.1",
+      "resolved": "https://registry.npmjs.org/bignumber.js/-/bignumber.js-9.3.1.tgz",
+      "integrity": "sha512-Ko0uX15oIUS7wJ3Rb30Fs6SkVbLmPBAKdlm7q9+ak9bbIeFf0MwuBsQV6z7+X768/cHsfg+WlysDWJcmthjsjQ==",
+      "license": "MIT",
+      "engines": {
+        "node": "*"
+      }
+    },
     "node_modules/buffer-from": {
       "version": "1.1.2",
       "resolved": "https://registry.npmjs.org/buffer-from/-/buffer-from-1.1.2.tgz",
@@ -797,6 +807,16 @@
         "node": "^8.16.0 || ^10.6.0 || >=11.0.0"
       }
     },
+    "node_modules/polyclip-ts": {
+      "version": "0.16.8",
+      "resolved": "https://registry.npmjs.org/polyclip-ts/-/polyclip-ts-0.16.8.tgz",
+      "integrity": "sha512-JPtKbDRuPEuAjuTdhR62Gph7Is2BS1Szx69CFOO3g71lpJDFo78k4tFyi+qFOMVPePEzdSKkpGU3NBXPHHjvKQ==",
+      "license": "MIT",
+      "dependencies": {
+        "bignumber.js": "^9.1.0",
+        "splaytree-ts": "^1.0.2"
+      }
+    },
     "node_modules/randombytes": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/randombytes/-/randombytes-2.1.0.tgz",
@@ -936,6 +956,12 @@
         "source-map": "^0.6.0"
       }
     },
+    "node_modules/splaytree-ts": {
+      "version": "1.0.2",
+      "resolved": "https://registry.npmjs.org/splaytree-ts/-/splaytree-ts-1.0.2.tgz",
+      "integrity": "sha512-0kGecIZNIReCSiznK3uheYB8sbstLjCZLiwcQwbmLhgHJj2gz6OnSPkVzJQCMnmEz1BQ4gPK59ylhBoEWOhGNA==",
+      "license": "BDS-3-Clause"
+    },
     "node_modules/supports-preserve-symlinks-flag": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/supports-preserve-symlinks-flag/-/supports-preserve-symlinks-flag-1.0.0.tgz",