[LogicalError]

Closes #238. Follows the tile picker section of the surface tools brief and the answers to the questions about it (pins stay per generator surface, variant pages pin, inside corner pieces never fit, decals get their own picker, only hotspot surfaces are counted).

Tile picker

• Materials: with more than one hotspot material on the surfaces, a row of buttons switches between them. A pin goes to that material's surfaces only (surfaces with ordinary materials are never changed), in one undo step, and spreads to connected surfaces as edits do.
• Fit states from matching (HotspotMatcher.Fits): fits all, fits some with an "n/m" count, doesn't fit (dark, not clickable), and a pin mark for Disabled and Alternate tiles. Inside corner pieces always count as not fitting. A hovered or pinned bordered tile shows its inside corner pieces dashed, and the info row names them; hovering the pieces names the tiles using them.
• "Automatic tile selection" (highlighted while active), find by name, Fit / 1:1, wheel zoom and middle drag pan, arrow keys and Enter.
• Pages: Base, variants (a preview; clicking pins, matching still picks the variant), other sheets.
• Legend with a summary, info row (name, pixels, aspect, flags, how many surfaces it fits) and Edit layout.
• Resizable from its corner, 600 wide by default, at least 420 x 420, its size remembered per user.
• Follows the surfaces live, and stays open so each material can be chosen in turn.
• Opens from: the hotspot tile field in the surface inspector and the UV tool (its text says what each material keeps, "B2 / Automatic"), and from Choose tile... in the Surface Browser's hotspot box.

Decal image picker

Replaces the decal inspector's use of the tile picker:

• the material's own texture over a checkerboard, every tile can be chosen
• Whole texture, find by name, Fit / 1:1, Edit layout
• picking sets every selected decal in one undo step
• resizable, 460 wide by default, at least 320 x 320, remembered

Differences from the brief

• No "Sheet of" label, no "Applies to N of M" and no Fitting only; the materials row only with more than one hotspot material.
• The info row says "Fits 3 of 4", without "the others stretch": matching places a pinned tile on a surface it doesn't fit, but whether it stretches there hasn't been checked.
• Both pickers stay open after a pick; the brief doesn't say.

Also fixed

• Hotspot diagnostic colours with the overlay closed: the view mode saved in the editor's preferences kept tinting hotspot surfaces even when the Chisel Hotspot Diagnostics overlay wasn't shown. They are now drawn only in a scene view that shows the overlay.
• Overlapping rows in Surface Options: in a narrow overlay the surface flags go one to a row, but the material block kept the height for two to a row, so Receive Shadows and Exclude From GI were drawn over the physics material and the surface flags. The block now takes the height the flags need.

Tests

New fixtures: HotspotTileChoiceTests, HotspotSheetViewTests, DecalImagePickerTests, SurfaceOptionsLayoutTests. All EditMode tests pass except ChiselGeneratorSizeTests.SpiralStairsFilledDown_HaveWallsDownToTheFloor (Up, Down), known failures tracked in #252.