Remove wall spots

Wall spots were markers for where SteamVR panels/windows could dock on a
home's walls (Meta's WallPlacementComponent). Removed the feature end to
end: the WorldWallSpot type and its JSON/binary serialization, the loader
that read WallPlacementComponent, auto-generation of spots and the hotspots
placed in front of them, the Studio editor's Wall Spot kind (Add menu,
properties, overlay draw, picking, selection, report), the debug overlay,
the home report counts, and the walktest viz. The PanelDock subsystem
(src/panel_dock.h) existed only to dock panels onto wall spots and is
deleted with its use in Frame Home's frame loop.

Baked scene format is bumped to v14: new bakes omit wall spots, and older
baked scenes still load (the wall-spot block is read past). Collision walls
(WorldCollider::Wall) are unaffected — only the panel-placement markers are
gone. Docs updated.

Note: the pre-existing FRAME_HOME_PC Windows build break in walkgrid.h
(the `near` macro) is unrelated and left as-is.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/studio-guide.md

@@ -114,7 +114,6 @@ edges, grey are floors you can't reach. **W** shows or hides it.
 | **Seat** | a place to sit: point at it and pull the trigger; its "stand up here" ring is where you get up |
 | **Teleport Hotspot** | a spot the teleport arc snaps to |
 | **Surface** | a box you can scale into a floor, a step or a wall |
-| **Wall Spot** | where SteamVR's panels dock |
 | **Invisible Wall** | stops you; never drawn on the headset. Scale and turn it to fit; where it stands is no longer walkable |
 | **Gravity Zone** | inside it, "down" is the zone's own: a direction, or any surface you stand on |