LCamDev: implement configureSessionModeCReq

We can, theoretically, now change the v4l camera's mode.

docs/lcamera-dev-lib.md

@@ -36,8 +36,10 @@ Channel splitting, colourspace conversion, threshold masks, and stencils belong
 in a separate shared raster library (`rasterStimulus`, future) and in
 `lcameraBuff`; `lcameraDev` stops at selector resolution, `CameraManager`
 lifecycle, and a refcounted, **acquired** `libcamera::Camera` handle per
-resolved device. Stream negotiation, pixel-format selection, frame buffers,
-and capture timing belong in `lcameraBuff` (and supporting libraries), not here.
+resolved device. Session mode negotiation (width, height, colour-space,
+fully-planar YUV requirement) happens in `lcameraDev` before capture starts.
+Frame buffers, request queues, and capture timing belong in `lcameraBuff` (and
+supporting libraries), not here.
 
 ## Why libcamera (for now)
 
@@ -228,9 +230,39 @@ Rules:
   the same physical device from SMO’s perspective; channel differences come from
   the qualeIface name on each DAP line.
 
-Streaming, frame delivery, and colourspace work are **out of scope** for
-`lcameraDev`; `lcameraBuff` uses the session’s acquired camera handle to set up
-capture on attach.
+## Session mode configuration (Stage 1)
+
+Before `lcameraBuff` starts capture, each producer calls
+`lcameraDev_configureSessionModeCReq` on the shared `CameraSession` with the
+requested mode:
+
+| Field | Role |
+|---|---|
+| `width` / `height` | Requested capture dimensions (non-zero) |
+| `colourSpace` | Semantic colour model (`Yuv` in v1) |
+| `fullPlanarIsOptional` | Default `false` — must select fully planar YUV |
+
+`lcameraDev` chooses a concrete libcamera pixel format (e.g. YUV420) from the
+camera’s supported formats. DAPS lines name the semantic colour-space, not a
+raw fourcc.
+
+Rules:
+
+* Configure only while the session exists and capture has not started.
+* **Identical** configure request on an already-configured session is a **no-op**
+  (multiple `lcameraBuff` producers sharing the same device selector each call
+  get-or-create then configure with the same mode).
+* **Different** configure request on an already-configured session throws
+  (conflicting mode requests on the same physical camera).
+* `fullPlanarIsOptional == false` (default): must select a fully planar YUV
+  format or throw with candidate-format diagnostics.
+* `fullPlanarIsOptional == true`: rejected at the configure API until
+  `lcameraBuff` implements non-planar producer deinterleaving (Stage 2). The
+  policy helper still accepts optional planar selection for future use.
+
+Streaming, frame delivery, and per-frame colourspace work remain **out of scope**
+for `lcameraDev`; `lcameraBuff` uses the configured session to allocate buffers
+and start capture on attach.
 
 ## dlopen API
 
@@ -273,8 +305,42 @@ typedef sscl::co::ViralNonPostingInvoker<void>
 
 Failures throw `std::exception`. `lcameraBuff` holds the returned
 `shared_ptr<CameraSession>` and passes it back to `releaseDeviceCReq`. The
-session wraps the acquired `libcamera::Camera`; higher layers configure and
-stream from that handle — `lcameraDev` does not expose frame or stream APIs.
+session wraps the acquired `libcamera::Camera`; higher layers configure the mode
+then stream from that handle.
+
+### Session mode configuration
+
+```cpp
+enum class LcameraDevColourSpace { Yuv };
+
+struct LcameraDevCameraModeRequest
+{
+    unsigned width = 0;
+    unsigned height = 0;
+    LcameraDevColourSpace colourSpace = LcameraDevColourSpace::Yuv;
+    bool fullPlanarIsOptional = false;
+};
+
+struct LcameraDevConfiguredCameraMode
+{
+    unsigned width;
+    unsigned height;
+    LcameraDevColourSpace colourSpace;
+    std::string pixelFormatName;
+    bool isFullyPlanar;
+    unsigned planeCount;
+};
+
+typedef sscl::co::ViralNonPostingInvoker<LcameraDevConfiguredCameraMode>
+    lcameraDev_configureSessionModeCReqFn(
+        const std::shared_ptr<CameraSession>& deviceSession,
+        const LcameraDevCameraModeRequest& request);
+```
+
+`lcameraDev_configureSessionModeCReq` delegates to
+`CameraSession::configureSessionModeCReq`, which runs libcamera
+`generateConfiguration` + `configure` and stores the result on the session.
+Identical reconfigure is a no-op; conflicting reconfigure throws.
 
 ### Enumeration (discovery)
 
@@ -298,6 +364,10 @@ When built with `-DENABLE_LIB_lcameraDev=ON`:
   `ComponentThread`.
 * `lcameraDev_probe <deviceSelector>` — `getOrCreateDeviceCReq`, then
   `releaseDeviceCReq` (selector and session attach/detach only).
+* `lcameraDev_configure_probe <deviceSelector> <width> <height> [options]` —
+  `getOrCreateDeviceCReq`, `configureSessionModeCReq`, print resolved mode (or
+  exception), then `releaseDeviceCReq`. Options: `--colour-space=yuv`,
+  `--opt-planar` / `--full-planar-is-optional`, `--reconfigure-twice`.
 
 ## Module layout
 
@@ -310,7 +380,8 @@ commonLibs/lcameraDev/
     cameraIdentity.h / .cpp             Discovery identity records
     selectorParse.h / .cpp              Compound selector parsing
     selectorResolve.h / .cpp            AND-match resolution
-    tools/                              lcameraDev_list_cameras, lcameraDev_probe
+    tools/                              lcameraDev_list_cameras, lcameraDev_probe,
+                                        lcameraDev_configure_probe
 ```
 
 Build links against `libcamera` (pkg-config). Does **not** link Salmanoff