v0.3.0: display abstraction, map-browser menu, SAGE terrain tiling/blends

- ra3.client::display: shared interactive loops; SDL and Vulkan backends
  implement only the primitives (init/present/poll_event/window_size/
  key_down/present_terrain). ra3.display picks the backend.
- Menu: maps by localized name (gamestrings.csf), red/gold theme, hover
  highlight, mouse + keyboard, wheel scroll, fullscreen and FPS/vsync
  options, loading progress bar.
- Terrain: continuous tile sampling via a texture array (REPEAT, uv =
  cell/(2*cellSize)) removes per-cell grid seams; SAGE blend ramp for
  material transitions; FPS label + top-right minimap overlays.
- Skip the skirmish sim for map views; reuse the Vulkan texture; no idle
  terrain redraw.

docs/REVERSE_ENGINEERING.md

@@ -78,6 +78,27 @@ Each holds an object pointer (0 when the subsystem is down).
   cash `+0x64`, player slots `+0xfc` (stride `0x5c`, 6 slots), faction at
   `slot + 0x18` (`Empire=2`, `Allied=4`, `Soviet=8`, `Random=7`).
 
+### Tactical view (camera)
+
+The in-game camera is the `TheTacticalView` object, held in the global at
+`0x00cdb7b4`. Its vtable accessors return zoom, pitch (current/target), yaw,
+world position and FOV; the debug overlay `FUN_005ef0a0` prints them through the
+format string at `0x00c0b900`. Mouse state is the singleton at `0x00ce9284`
+(cursor position vtable slot `+0x3c`, button down `+0x48`); the keyboard manager
+is `0x00ce927c` (modifier mask `+0x38`).
+
+Per-map camera tuning is a named-field table in `.rdata` (around
+`0x00c11a54`): `cameraMinHeight`, `cameraMaxHeight`, `cameraPitchAngle`,
+`cameraYawAngle`, `cameraScrollSpeedScalar`, `cameraGroundMinHeight`,
+`cameraGroundMaxHeight`. The map-load chunk `CHUNK_TacticalView`
+(`0x00beea14`, consumed near `0x00548a00`) seeds the view from the map; the
+tutorial actions `LOCK_CAMERA_SCROLL` / `LOCK_CAMERA_ZOOM` / `LOCK_CAMERA_ROTATION`
+gate the controls.
+
+OpenRA3 has no 3D terrain yet, so `ra3::render::view_camera` reproduces the
+*controls* - wheel zoom, screen-edge scroll, clamped pan and opening on the
+player's start - over the 2D map overview, not the retail perspective camera.
+
 ### Match start
 
 The BEGIN button calls `SkirmishGameOptionsMenu::start` (`0x00b28d60`), which
@@ -127,9 +148,84 @@ in `MPPositionList` instead yield no waypoints and fall back.
 Verified example (`map_mp_2_feasel4`): `Player_1_Start` = `(1338.9, 1940.5, 0)`,
 `Player_2_Start` = `(1290.8, 1404.9, 0)`.
 
+### Terrain (`ra3.terrain`)
+
+The `CkMp` tree is a flat chunk list: `"CkMp"`, `u32 assetCount`, the name
+table (`{ u8 len, name, u32 index }`, index descending from `assetCount`), then
+`{ u32 index, u16 version, u32 size, data }` per chunk. On
+`map_mp_2_feasel4` the terrain is `HeightMapData` v6 (540 x 600, border 20,
+`u16` elevations, scale `0.0390625`) and `BlendTileData` v27.
+
+`BlendTileData` opens with `NumTiles`, the `u16` tile grid, then the
+blend/three-way/cliff tables (`u16` for v27); the passability flag arrays that
+follow are not needed for rendering, so the texture table is located by
+scanning for its `{ cellStart, cellCount, cellSize, magic }` + `u16`-prefixed
+name entries. A tile value is `(cellIndex << 2) | variant`, and `cellIndex`
+indexes the global `TextureCellCount`-cell table (each texture owning
+`cellSize^2` 64 px cells). The textures themselves are
+`art\terrain\<stem>.tga` in `Terrain.big` / `Core11.big` (RefPack + 256x256
+TGA). Rendered top-down, `map_mp_2_feasel4` correlates 0.94 with the official
+`_art.tga` overview.
+
+### Terrain blending (`BlendTileData` tail)
+
+After the tile grid, `BlendTileData` stores three per-cell `u16` tables —
+`Blends`, `ThreeWayBlends` and `CliffTextures` — then `TextureCellCount`,
+`BlendsCount`, the texture table, two magic words and `BlendsCount - 1` blend
+descriptions (18 bytes each: `u32 secondaryTile`, four direction bytes,
+`u8 flags`, `u8 twoSided`, `u32 0xFFFFFFFF`, `u32 0x7ADA0000`). A non-zero
+`Blends[cell]` is a 1-based index into the descriptions; `secondaryTile` is a
+packed tile value (`secondaryTile >> 2` is its cell).
+
+The retail `Terrain.fx` (compiled `terrain.fxo`, parameters `Terrain.BaseTexture`,
+`Terrain.MacroTexture`, `MapCellSize`, `IsTerrainAtlasEnabled`; technique
+`TerrainTile`) cross-fades a cell's base tile into `secondaryTile` with a linear
+ramp selected by `BlendDirection`: `1` right, `2` top, `4` top-right, `8`
+top-left, where `flags` bit 0 flips the axis and bit 1 marks a two-sided
+diagonal. OpenSAGE's `Terrain.frag` reconstructs the exact
+`CalculateBlendFactor`; `ra3::terrain::blend_factor` and `shaders/terrain.frag`
+mirror it. The row axis is *not* inverted (73% of long-axis blends point at a
+neighbour of the same texture, versus 25% inverted). On `map_mp_2_feasel4`,
+33828 of 324000 cells carry a blend and there are 7094 descriptions.
+
+The GPU atlas pads every 64 px tile with a 2-texel replicated gutter. Packed
+edge-to-edge, bilinear/mipmap filtering averaged two unrelated tiles at every
+cell border — that cross-tile bleed was the visible grid line the hardware path
+drew. (`shaders/terrain.frag` samples `cell_stride = cell_texels + 2 * gutter`.)
+
+### Open question: per-cell tile sampling
+
+Measured on `map_mp_2_feasel4`, sampling each cell as its own 64 px block and
+restarting the UV every cell leaves a 1.39x edge spike at cell boundaries
+(43.4 vs 31.1 mean gradient at 8 px/cell). Two candidate mappings reduce it and
+need a visual decision against the retail art:
+
+| Mapping | Boundary/interior |
+| --- | --- |
+| per-cell 64 px block (current) | 1.39 |
+| OpenSAGE `BlendTileTextureIndex` Morton layout, 32 px block | 1.18 |
+| continuous `uv / (cellSize * 2)`, per OpenSAGE `Terrain.frag` | 1.11 |
+
+The retail `Terrain.frag` (OpenSAGE) samples the tile texture *continuously*
+(`uv / (CellSize * 2)`), so adjacent cells never restart the texture; our
+per-cell restart is the remaining source of grid lines. Switching the atlas
+from 64 px blocks to continuous 32 px regions (or the Morton 8x8 layout) is the
+next step, pending an art correlation check.
+
+### Map display names
+
+The skirmish map list labels live in `Data\English.big`'s
+`data\gamestrings.csf` (the newest `Lang-English*.big` wins) under
+`MAP:<UPPERCASE_ID>`, e.g. `MAP:MAP_MP_2_FEASEL4` = "Battlebase Beta". CSF
+values are UTF-16 code units whose low byte is XORed with `0xFF`
+(`ra3::map::parse_map_names`). `openra3 extract` writes the decoded table to
+`maps/map_names.tsv`; `openra3 menu` shows them instead of the raw map id.
+
+
 ### Still to recover
 
 - `MPPositionList` layout (per-player starts for maps without waypoints).
-- Map dimensions / `HeightMapData` / `BlendTileData`.
-- Compiled asset blobs (`map.bin`, `global.bin`, `static.*.bin`) and the
-  `.manifest` schema used to deserialise them.
+- Cliff textures and the `CliffTextureMapping` UV remap (`CliffTextures` is
+  parsed but not yet drawn).
+- Compiled asset blobs (`global.bin`, `static.*.bin`) and the `.manifest`
+  schema used to deserialise them.