daemon: AV1 Sequence Header OBU synthesiser + unit test

V4L2 stateless AV1 passes the sequence header information as a
structured control (V4L2_CID_STATELESS_AV1_SEQUENCE) and ships
only tile-group bytes in the OUTPUT buffer.  libavcodec's AV1
decoder is full-bitstream, so the daemon needs to reconstruct
the OBU bytes the consumer parsed out before feeding the
assembled stream to libavcodec.

This commit lands the Sequence Header OBU half of that
reconstruction — av1_synth_sequence_header_obu().  Frame
Header / Frame OBU synthesisers + the integration that wires
the assembled OBUs into the decode hot path are separate
follow-on modules.

Module shape mirrors the H.264 NAL synthesiser (PR #1):

  - Public API: single function returning byte count or 0
    on overflow/invalid input.
  - Wire encoder uses the existing bitstream_writer (bsw_put_u
    is AV1's f(n); bsw_put_ue is bit-identical to AV1's uvlc;
    bsw_align_rbsp matches AV1's trailing_bits()).
  - AV1-specific helpers (leb128 size, min_bits_for, subsampling
    resolution per §5.5.2) are file-local statics.
  - No emulation prevention — AV1 uses leb128-sized OBUs for
    bitstream boundaries, not byte-pattern escapes.

Synthesis decisions for fields V4L2 doesn't carry are documented
verbatim in the file header (reduced_still_picture_header = 0;
single operating point at seq_level_idx = 13 / level 5.1;
color_description_present_flag = 0; chroma_sample_position = 0;
seq_choose_screen_detection_tools = 1; seq_choose_integer_mv = 1).

Rejection cases:
  - seq_profile > 2
  - bit_depth not in {8, 10, 12}
  - seq_profile = 1 + monochrome (4:4:4 forced colour)
  - seq_profile = 1 + bit_depth = 12 (only profile 2 allows it)
  - max_frame_{width,height}_minus_1 requiring > 16 length bits
  - out_cap too small to hold header + leb128 + payload

Each returns 0 to surface the mismatch loudly rather than emit
nonsense the libavcodec parser would reject downstream.

Unit test (test_av1_obu_synth.c, opt-in via DAEDALUS_BUILD_TESTS=ON)
exercises four cases bit-by-bit against a hand-computed reference:

  1. profile 0, 1080p, 8-bit, 4:2:0, order_hint on (7 bits),
     CDEF+restoration on — the common Pi 5 path.
  2. profile 0, 720p, 10-bit, monochrome — exercises high_bitdepth
     and the monochrome short-form color_config.
  3. profile 1 + bit_depth 12 → expects 0 (rejected).
  4. tiny out_cap → expects 0 (overflow).

All four green on hertz (aarch64 Arch, gcc Wall+Wextra+Wpedantic
clean).

This commit does not change daemon behaviour — av1_obu_synth.c is
built into the daemon binary so the symbols are reachable, but
no call site is wired yet.  Integration goes in the follow-on
DAEMON-AV1 patches that also synthesise the Frame Header OBU
and bracket the assembled OBUs with a Temporal Delimiter.

Refs reauktion/daedalus-v4l2#11 daemon-half; closes daedalus
backlog task #144.

daemon/CMakeLists.txt

@@ -52,6 +52,7 @@ add_executable(daedalus_v4l2_daemon
 	src/dmabuf_capture.c
 	src/bitstream_writer.c
 	src/h264_nal_synth.c
+	src/av1_obu_synth.c
 )
 
 target_include_directories(daedalus_v4l2_daemon
@@ -80,3 +81,25 @@ target_link_libraries(daedalus_v4l2_daemon
 
 install(TARGETS daedalus_v4l2_daemon
 	RUNTIME DESTINATION /usr/local/bin)
+
+# --- Unit tests (opt-in) -------------------------------------------------
+#
+# DAEDALUS_BUILD_TESTS=ON enables standalone test executables that run on
+# the build host (no V4L2 / FFmpeg / Vulkan dependency).  Used by CI to
+# gate bitstream synthesis modules against regressions.
+
+option(DAEDALUS_BUILD_TESTS "Build daemon unit tests" OFF)
+
+if (DAEDALUS_BUILD_TESTS)
+	add_executable(test_av1_obu_synth
+		src/test_av1_obu_synth.c
+		src/av1_obu_synth.c
+		src/bitstream_writer.c
+	)
+	target_include_directories(test_av1_obu_synth PRIVATE src)
+	# Test binary does not link FFmpeg / Vulkan / dl — it exercises
+	# pure-C encoders against in-memory inputs.
+
+	enable_testing()
+	add_test(NAME av1_obu_synth COMMAND test_av1_obu_synth)
+endif()