iter2 step3: HEVC EXT_SPS_*_RPS UAPI header + runtime probe

src/hevc-ctrls/v4l2-hevc-ext-controls.h (NEW, MIT, ~95 LOC):
  Verbatim mirror of Linux 7.0 V4L2_CID_STATELESS_HEVC_EXT_SPS_ST_RPS
  and _LT_RPS control IDs + struct definitions + flag macros. Each
  symbol is ifndef-guarded so when ampere's linux-api-headers
  eventually bumps to 7.0+, the kernel header takes precedence and
  this shim silently no-ops. Citation block links the upstream
  Casanova v8 series.

  Per LGPL section 3.b, kernel UAPI struct definitions are excepted
  from GPL inheritance, so copying them into MIT userspace is fine.

src/request.h: added has_hevc_ext_sps_rps_rkvdec + _hantro bool
  fields on struct request_data — pair-of-flags layout mirrors
  video_fd_rkvdec / video_fd_hantro (iter38 multi-device-probe
  pattern, per feedback_multi_device_probe_design). Phase 5 review
  identified single-scalar storage as a silent-misbehavior risk
  across device-switch boundaries.

src/request.c:
  - new probe_hevc_ext_sps_rps_controls(fd) helper: queries the two
    new CIDs via VIDIOC_QUERYCTRL; returns true iff both register.
    RK3399 rkvdec (linux 6.x or 7.x without VDPU381/383 bindings)
    returns false; RK3588 rkvdec (VDPU381/383) returns true.
  - probe each driver_data->video_fd_rkvdec / _hantro after the
    iter38 multi-device-probe block at VA_DRIVER_INIT time
  - log-line if rkvdec supports it - diagnostic for Phase 7

src/meson.build: added the new UAPI header to the headers list.

Build verified: ninja -C build clean, .so produced. The new probe
runs at driver init and stores the result, but nothing CONSUMES the
result yet — that's Step 4 (h265_set_controls wiring).

Per ampere-kernel-decoders campaign iter2 Phase 4 step 3 (amended
by Phase 5 review item 'per-fd storage').

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

src/meson.build

@@ -89,6 +89,11 @@ headers = [
 	'codec.h',
 	'nv15.h',
 
+	# Internal mirror of Linux 7.0 V4L2 HEVC EXT_SPS_*_RPS UAPI defs
+	# (allows building against pre-7.0 linux-api-headers; redundant
+	# once the host headers are 7.0+).
+	'hevc-ctrls/v4l2-hevc-ext-controls.h',
+
 	# Vendored GStreamer + project shim headers (see sources above).
 	'h265_parser/gst_compat.h',
 	'h265_parser/gst/gst.h',