daemon: shadow_decoder wiring (PR-Q3a.1)

Toolchain plumbing for the upcoming daedalus-decoder shadow-mode
path.  Production behaviour is unchanged.

What lands here:

  1. CMake links libdaedalus_decoder via pkg-config.  Static archive,
     so no .so dependency change in the daemon's link map.
  2. ffmpeg_loader resolves ff_h264_set_mb_inspect_cb NULL-tolerantly.
     Stock libavcodec lacks the symbol (logged as INFO at startup);
     the marfrit-packages ffmpeg-v4l2-request-fourier fork's 0016
     patch exports it.  The shadow path activates only when both
     env DAEDALUS_SHADOW_MODE=1 AND the symbol resolves.
  3. New shadow_decoder.[ch] module:
       - shadow_decoder_create() gates on env + symbol presence,
         returns NULL in production state (the common case).
       - shadow_decoder_install_cb() registers a per-MB callback on
         the H.264 AVCodecContext; lazily-created daedalus_decoder
         context will pick up dimensions from the first AVFrame.
       - shadow_decoder_on_frame() logs per-frame MB-observed count.
     Every entry point is NULL-safe so decoder.c stays clean of
     conditionals.
  4. decoder.{c,h} grow a `struct shadow_decoder *shadow` field on
     daedalus_decoder.  Install hook fires once per H.264 codec open;
     frame hook fires after each successful avcodec_receive_frame.

PR-Q3a.1 scope ENDS here.  The callback just counts MBs; no
daedalus_decoder_append_mb or flush_frame yet.  Real-coeffs / edges
extraction needs the patched FFmpeg source-tree headers
(DAEDALUS_FFMPEG_SRC) to introspect H264Context internals — that
lands in PR-Q3a.2.

dejavu-check: this path is daedalus-decoder's frame-major UMA
dispatch architecture (one cmdbuf per frame, one submit) running
alongside libavcodec's reference decode for validation.  It is NOT
per-kernel libavcodec function-pointer substitution.  No new
libavcodec patches; the existing 0016 callback is the only intercept
point.

Verified on hertz:

  - Build: clean, libdaedalus_decoder.a linked.
  - Disabled state (env unset OR symbol absent): no shadow log
    lines, daemon init continues normally, INFO logs
    "libavcodec lacks ff_h264_set_mb_inspect_cb (stock build,
    no daedalus-fourier 0016 patch) — shadow-mode unavailable".
  - Enabled state would require ffmpeg-v4l2-request-fourier .deb
    rebuilt with patches 0016/0017 deployed to hertz (current .deb
    release 10 predates them).  That's a deployment task, separate
    from this PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

daemon/CMakeLists.txt

@@ -40,6 +40,11 @@ pkg_check_modules(FFMPEG REQUIRED IMPORTED_TARGET
 # libdaedalus_core.a must precede -lvulkan because the static archive
 # references vulkan symbols and the linker resolves left-to-right.
 pkg_check_modules(DAEDALUS_FOURIER REQUIRED daedalus-fourier)
+# daedalus-decoder — frame-major UMA H.264 decoder.  Linked into the
+# shadow-mode path (env DAEDALUS_SHADOW_MODE=1) and inert otherwise.
+# Linked unconditionally to keep CMake configurations symmetrical
+# between production and shadow-mode runs.
+pkg_check_modules(DAEDALUS_DECODER REQUIRED daedalus-decoder)
 find_package(Vulkan REQUIRED)
 
 add_executable(daedalus_v4l2_daemon
@@ -48,6 +53,7 @@ add_executable(daedalus_v4l2_daemon
 	src/log.c
 	src/parser.c
 	src/decoder.c
+	src/shadow_decoder.c
 	src/chardev_client.c
 	src/dmabuf_capture.c
 	src/bitstream_writer.c
@@ -61,20 +67,25 @@ target_include_directories(daedalus_v4l2_daemon
 		${CMAKE_CURRENT_SOURCE_DIR}/../include
 		${FFMPEG_INCLUDE_DIRS}
 		${DAEDALUS_FOURIER_INCLUDE_DIRS}
+		${DAEDALUS_DECODER_INCLUDE_DIRS}
 )
 
 # dl for dlopen, pthread for future threading work.
 target_link_directories(daedalus_v4l2_daemon
 	PRIVATE
 		${DAEDALUS_FOURIER_LIBRARY_DIRS}
+		${DAEDALUS_DECODER_LIBRARY_DIRS}
 )
 
 target_link_libraries(daedalus_v4l2_daemon
 	PRIVATE
 		dl
 		pthread
-		# Order matters: libdaedalus_core.a first (so its undefined
-		# vulkan symbols register), then -lvulkan to satisfy them.
+		# Order matters for left-to-right linker resolution of
+		# static archives.  daedalus-decoder references symbols
+		# from daedalus-fourier; daedalus-fourier references
+		# vulkan symbols.  So: decoder, fourier, vulkan.
+		${DAEDALUS_DECODER_LIBRARIES}
 		${DAEDALUS_FOURIER_LIBRARIES}
 		Vulkan::Vulkan
 )