feat: a Tools menu, a Build list, mesh tools and bridge fixes

[raighen]

Based on main. This is the first half of #10: everything in it except the loading zones, which stay in #10 as a draft on top of this.

What

• Tools menu. Windows over jobs that only the MCP server could do: Hide triangles, Mirror to winter, Stream map, Clone / Replace / Export car.
• Hide triangles. Cuts an opening in a stock mesh without rebuilding it. With the window open the triangles are picked by clicking them on the mesh in the viewport, or by a box for many at once; what would be hidden is outlined first, and the count says what goes on each level of detail. A mesh whose geometry other objects draw too is cut alone (it gets a geometry block and buffers of its own first, as a duplicate does) or together with them. mesh_hide_triangles does the same with shared: own | all.
• Build list. Build opens a window first: each edited archive with a tick and the files of its working copy that differ from the archive the game has. Files written before the session are marked. "Add archive" packs a working copy changed by hand.
• Mesh tools. mesh_materials re-points a slot, collision_unused_hulls and crash_placements clean up, scene_find finds a collision placement by its hull.
• Bridge. A skinned mesh can gain vertices (its remap pools are rebuilt), two levels of one object go in one push, a person's mesh is no longer turned twice in Blender.
• Library browser. Pictures of characters, wardrobe pieces and cars.
• Fix. A resize of the viewport while a district loads no longer ends the program.

How it was checked

• Illusion.slnx builds clean, dotnet format clean on the touched files.
• Probes, passed/failed: hide-triangles 25/0, build-list 23/0 (46/0 over six archives), remap-pools 105/0, archive-thumbs 9/0, editor-tools 23/0, mcp 66/0, material-editor 65/0, season-mirror 26/0, object-transplant 65/0, bridge-material 69/0, car-clone 77/0.
• This code was gone over twice with your findings on feat(mcp): tools that drive the map editor, actor import across archives, light placement #3 to fix: an imported object brings textures its own archive does not hold #9 as the checklist, by a reader who had not written it; what that found is fixed here (the Build list missing a deleted resource or a deleted Mipmap, a comparison failure locking the Build button, a slot of a shared material block re-pointed for every object, and smaller ones).
• By hand in the running editor: triangles picked by clicks on a mesh that shares its geometry, hidden on it alone and undone, and the same through mesh_hide_triangles with shared set to own and to all; Build pressed in the Build window and the built archive compared with its working copy; Export car run from its window; Clone car and Mirror to winter up to their refusals.

Limits

• A click picks triangles of the level of detail drawn up close. On a coarser level only the triangles lying on the picked ones go with them; where a level is cut coarser it stays whole, and the window says so. The box takes every level.
• A slot of a material block several objects draw with is not re-pointed; the refusal names them.
• The first box query of scene_find decodes each distinct collision hull once.
• A picture of an archive that was never opened extracts it first.