BMD editor: editing, saving, and a real details UI

[robert-d-schultz]

The BMD editor was read-only: it parsed a .bmd and rendered a text dump of every field. There was no writer at all, so nothing could be saved.

Format layer:

• Add BmdWriter, mirroring BmdParser section-for-section over the whole file (not just the editable categories), with a self-check that re-parses its own output before returning. Legacy/undocumented branches that the parser itself discards throw NotSupportedException rather than silently corrupting data.
• Capture data the parser previously read purely to advance the stream and then dropped, which a writer cannot reproduce otherwise: BmdFile.SectionVersions, PropInfo.PropIndex (prop string tables contain duplicate paths, so re-deriving an index can silently repoint a prop), and CultureMask.RawBytes (several bit positions map to no named field and would be zeroed on reconstruction).
• Fix BmdParser never populating BmdFile.Props (the shared prop string table was read and thrown away).
• Fix unreachable spotlight branch: if (Version > 3) ... else if (Version > 4) meant v>4 spotlights read a UInt32 PdlcMask instead of a UInt64, misaligning the stream by 4 bytes.

Editor:

• Implement ISaveableEditor with a Save button and dirty tracking.
• Per-category transform editing, exposing only the degrees of freedom the format actually stores: full TRS for props/decals/VFX/composite scenes and polymesh v>3; position+rotation for spot lights; position only for point lights, light probes, terrain hole vertices, sounds, and polymesh v<=3 (bulk vertex offset). Transforms are edited as position / euler degrees / scale rather than raw matrices.
• Add BmdGizmoComponent (ported from CscGizmoComponent) driving the same properties, with rotate/scale gated to categories that support them.
• Replace the text-dump details panel with per-category templates of real controls, and make paths editable (prop model, VFX, sound event, polymesh material, composite scene file).
• Group the component tree into collapsible per-category sections and split decals out from props.
• Add an "Add" menu covering all ten categories, matching the CSC editor.
• Add Terry project export (.terry/.layer) via pack file context menu.
• Remove the unused BmdSceneView/Bmd3DSceneViewer/BmdSceneViewModel trio and the unreferenced BmdBmdReferenceKey class.

Verified: all 44 local .bmd files (36 campaign prefabs, 8 terrain tile_maps) round-trip byte-for-byte identical; edits through the view models change only the intended bytes; the ten Add-menu defaults write and re-parse correctly.