iter39 α-31: H264 Hi10P + HEVC Main10 sub-profile support (10-bit, rkvdec NV15)

Adds VAProfileH264High10 and VAProfileHEVCMain10 to the libva-v4l2-request
backend. RK3399 rkvdec emits decoded frames as V4L2_PIX_FMT_NV15 (4 × 10-bit
values packed in 5 bytes per element); VAAPI consumers receive standard
VA_FOURCC_P010 via a new userspace unpack in copy_surface_to_image.

VP9 Profile 2 explicitly NOT added — RK3399 rkvdec kernel ctrl table
caps at V4L2_MPEG_VIDEO_VP9_PROFILE_0 (rkvdec.c::rkvdec_vp9_ctrl_descs).

Touchpoints (per Phase 5 sonnet-architect review amendments):
  - include/drm_fourcc.h: define DRM_FORMAT_NV15 (vendored libdrm lacks it)
  - src/nv15.{c,h}: NV15 → P010 plane unpack (LSB-first, per
    Documentation/userspace-api/media/v4l/pixfmt-nv15.rst)
  - src/video.c: NV15 entry in formats[] (else NULL-deref on video_format_find)
  - src/codec.c: pixelformat_for_profile cases for Hi10P + Main10
  - src/config.c: enumeration, validation, entrypoints, RT_FORMAT_YUV420_10
    advertisement for 10-bit profiles
  - src/context.c: per-profile CAPTURE pix_fmt (NV12/NV15), 10-bit synthetic
    SPS (bit_depth_luma_minus8=2), video_format invalidation on bit-depth
    transition (sibling to iter38 device-switch invalidation), is_10bit flag
  - src/surface.c: RT_FORMAT_YUV420_10 admission, NV15 fourcc on PRIME export
  - src/image.c: P010 reporting in DeriveImage + QueryImageFormats,
    P010-aware sizing in CreateImage, NV15 → P010 unpack call in
    copy_surface_to_image (gated on is_10bit + image.format.fourcc == P010)
  - src/picture.c: 4 switch blocks route Hi10P/Main10 to existing H264/HEVC
    per-codec paths
  - src/request.h: MAX_PROFILES bump 11 → 13, driver_data->is_10bit flag

Scope: COPY path (vaGetImage / vaDeriveImage) only. Standard ffmpeg-vaapi
hwdownload, mpv vaapi-copy, and any consumer using vaGetImage works
end-to-end. PRIME-path consumers that only know NV12/P010 must use the
COPY path; PRIME consumers aware of NV15 (panfrost-Mesa et al.) get the
correct fourcc on RequestExportSurfaceHandle. PRIME-side P010 emission is
follow-up scope (would need DRM_FORMAT_P010 + per-plane unpack into a
GPU-accessible buffer).

Compile-tested on boltzmann (aarch64 native, gcc 15.2.1, libva 1.23.0,
libdrm 2.4.133): clean build, .so produced, 0 new warnings.

Phase 0/2 evidence: linux-mmind-v7.0 drivers/media/platform/rockchip/rkvdec.
rkvdec_h264_decoded_fmts[] and rkvdec_hevc_decoded_fmts[] both list NV15;
ctrl tables cap at HEVC MAIN_10 and H264 HIGH_422_INTRA (Hi10P < cap, not
in menu_skip_mask). image_fmt resolution (rkvdec-h264-common.c:196,
rkvdec-hevc-common.c:467) dispatches on bit_depth_luma_minus8 only.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

include/drm_fourcc.h

@@ -195,6 +195,11 @@ extern "C" {
 #define DRM_FORMAT_NV24		fourcc_code('N', 'V', '2', '4') /* non-subsampled Cr:Cb plane */
 #define DRM_FORMAT_NV42		fourcc_code('N', 'V', '4', '2') /* non-subsampled Cb:Cr plane */
 
+/* iter39: NV15 is 4×10-bit packed in 5 bytes (Rockchip rkvdec 10-bit output). */
+#ifndef DRM_FORMAT_NV15
+#define DRM_FORMAT_NV15		fourcc_code('N', 'V', '1', '5') /* 2x2 subsampled Cr:Cb plane 10 bits per channel packed */
+#endif
+
 /*
  * 3 plane YCbCr
  * index 0: Y plane, [7:0] Y