feat v1.42.0: room settings — the third settings tier

- four-tier principle fixed in ARCHITECTURE: global > space > room >
  device, specific overrides general, unset inherits
- room.settings { fill_mode, temp_source, hum_source } with backend
  schema; pure roomFillModeOf + sourceValue (+2 test suites, 114)
- gear on room cards in the Plan editor opens Room settings: rename,
  re-area (current area included in the list), fill override (may opt
  out of glow darkness), explicit temp/hum source with a searchable
  device+entity dropdown; the same section in the creation dialog
- source feeds the room card, tooltip and temp fill; works for rooms
  without an HA area (user-feedback case #1)
- smoke_room_settings.mjs (10 checks); TESTING/CHANGELOG/ARCHITECTURE
  same-commit

docs/ARCHITECTURE.md

@@ -243,3 +243,15 @@ hash falls back to the default.
   pipeline (`swipeTarget`), per-screen multipliers in `LS_KIOSK`.
 - **Nav persistence** (v1.38.2): `LS_NAV` stores {space, mode}; hash
   deep-link > saved > default_floor; stale-cache retry after the live load.
+
+
+## Settings tiers (owner's principle, 2026-07-26)
+
+Four levels: **global (config.settings) → space (space.settings) → room
+(room.settings) → device (marker.*)**. Duplicated options are deliberate: the
+more specific tier overrides the more general one; "unset" always means
+"inherit". Resolution lives in pure helpers (`spaceDisplayOf`,
+`roomFillModeOf`, `sourceValue`, `resolveTapAction`) — never inline in render.
+The UI will later be unified around this model; until then each tier keeps its
+own dialog (general settings gear / space gear / room-card gear / marker
+dialog).