v0.4.0: Direct3D 11/12 backends, in-game renderer selection, per-run logging

- ra3.dx: D3D11 and D3D12 presentation backends (runtime HLSL via d3dcompiler);
  2D image blit and the GPU heightfield terrain raymarch, with the corner
  minimap/FPS overlays. Non-Windows builds link a null fallback.
- ra3.display: preferred backend plus ordered fallback (Vulkan/D3D11/D3D12/SDL);
  menu gains a Renderer option and the CLI gains --dx11/--dx12/--sdl, which the
  render command now honours too.
- vendor libenderlog (MIT): every run writes openra3.log next to the exe and
  archives the previous run's log; records at warn and above carry a call stack
  (native fallback, since libc++ has no <stacktrace>).
- Windows crash reporter writes openra3_crash.log (faulting module + backtrace);
  D3D/DXGI diagnostics are routed through the logger.

docs/ARCHITECTURE.md

@@ -62,9 +62,9 @@ umbrella module re-exports the SDK; applications import `ra3` only.
                                   |             \        |
                                   |              \       v
                                   |             ra3.display (backend pick)
-                                  |              /        \
-                                  v          ra3.ui    ra3.vulkan
-                                  |          (SDL3)    (Vulkan)
+                                  |              /     |      \
+                                  v          ra3.ui   ra3.vulkan  ra3.dx
+                                  |          (SDL3)   (Vulkan)   (D3D11/12)
    ---------------------------------------------------------------------------
                                   v
    L2  simulation         ra3.logic  ra3.modules  ra3.combat  ra3.movement
@@ -80,7 +80,8 @@ umbrella module re-exports the SDK; applications import `ra3` only.
 
 The current concrete modules (`ra3.core`, `ra3.logic`, `ra3.data`,
 `ra3.skirmish`, `ra3.fs`, `ra3.map`, `ra3.terrain`, `ra3.render`, `ra3.ui.*`,
-`ra3.vulkan.*`, `ra3.display`, `ra3.game`, `ra3.client`) are the **seeds** of the
+`ra3.vulkan.*`, `ra3.dx.*`, `ra3.display`, `ra3.game`, `ra3.client`, and the
+vendored `ender.log`) are the **seeds** of the
 target modules below. `ra3.skirmish` and `ra3.game` will be absorbed into
 `ra3.ai` / `ra3.match`; new modules are added as their subsystems are recovered.
 
@@ -557,7 +558,8 @@ Mirrors SAGE `GameEngine/Common`. Everything else speaks this.
   - `[x]` window, streaming-texture blit, input polling
   - `[ ]` gamepad support `(v0.8)`
 - **F3 Backend selection (`ra3.display`)** `[D]`
-  - `[x]` try Vulkan → SDL → report failure for offscreen fallback
+  - `[x]` preferred backend + ordered fallback (Vulkan / D3D11 / D3D12 / SDL)
+  - `[x]` in-game Renderer option; report failure for offscreen fallback
 - **F4 Input mapping** `[~]`
   - `[x]` keyboard/mouse state, modifier masks
   - `[ ]` rebindable keybinds, mouse capture, scroll wheel events `(v0.6)`
@@ -578,6 +580,21 @@ Mirrors SAGE `GameEngine/Common`. Everything else speaks this.
 - **F4 Null fallback** `[D]`
   - `[x]` report failure when no Vulkan loader is present
 
+### M18b `ra3.dx` — Direct3D 11 / 12 backend `[P]`
+
+- **F1 Device & swapchain** `[D]`
+  - `[x]` SDL3 window → HWND, DXGI flip-model swapchain, resize
+  - `[x]` runtime HLSL via `d3dcompiler_47` (no build-time shader compiler)
+- **F2 2D image path** `[D]`
+  - `[x]` BGRA scene texture + fullscreen-triangle blit (D3D11/D3D12)
+- **F3 Terrain presentation** `[D]`
+  - `[x]` GPU heightfield raymarch (HLSL port of `terrain.frag`), D3D11 and D3D12
+- **F4 Root signatures / PSOs (D3D12)** `[~]`
+  - `[x]` root constants, descriptor tables, static samplers, barriers
+  - `[ ]` shared RHI with Vulkan (see M16 F1) `(v0.6)`
+- **F5 Null fallback** `[D]`
+  - `[x]` non-Windows builds link a stub that fails `init`
+
 ### M19 `ra3.audio` — audio `[ ]`
 
 - **F1 SFX** `[ ]`