Phase 8.3: userspace daemon scaffold + FFmpeg dlopen + parse path

Builds the daemon executable per the locked Phase 8 architecture
(Option γ: dlopen FFmpeg at runtime). Phase 8.3 scope: parse
path validation only — no V4L2 wiring, no decode, no chardev
connection.

Components:
- daemon/CMakeLists.txt — CMake with -Wall -Wextra -Wpedantic
  clean. pkg-config for FFmpeg headers; only -ldl + -lpthread
  at link time.
- daemon/src/main.c — entry point, signal handlers
  (SIGINT/SIGTERM), command dispatcher. Currently `parse <file>`.
- daemon/src/ffmpeg_loader.{c,h} — runtime FFmpeg loader.
  dlopens libavformat.so.61, libavcodec.so.61, libavutil.so.59.
  Resolves 22 function pointers using POSIX-recommended
  *(void**)& dlsym idiom (per POSIX.1-2017 dlsym(3p) Rationale).
- daemon/src/parser.{c,h} — demux loop via avformat_open_input +
  av_read_frame. Per-frame logging on -v.
- daemon/src/log.{c,h} — logging facade (stderr Phase 8.3;
  syslog/journal planned for 8.5+).

Verification on hertz:
  $ ffmpeg -f lavfi -i testsrc=duration=2:size=320x240:rate=30 \
           -c:v libvpx-vp9 -y /tmp/testsrc.ivf
  $ daedalus_v4l2_daemon parse /tmp/testsrc.ivf
  [INFO] FFmpeg loaded: 7.1.3-0+deb13u1+rpt1 (libavformat 61.7.100)
  [INFO] video stream #0: codec=vp9 (Google VP9) 320x240, 0/0 fps
  [INFO] parse complete: 60 frames (1 key) total 17859 bytes

Error paths verified:
- Missing file → "avformat_open_input(...): code -2", exit 1
- No command → usage message, exit 2
- Bad command → usage message, exit 2

Per correctness-before-speed:
- Real CMake (no Makefile hacks)
- pkg-config for headers
- POSIX-conformant dlsym pattern (no -Wpedantic suppression)
- Real signal handling + proper exit codes
- Real logging with timestamp + level
- Headers included at compile-time for type safety; dlopen
  decouples runtime
- All FFmpeg resources freed on every exit path
- Builds clean on -Wall -Wextra -Wpedantic

Phase 8.3 acceptance criteria met:
- ✓ daemon binary builds
- ✓ dlopen FFmpeg at runtime
- ✓ demux a VP9 IVF file end-to-end
- ✓ per-frame metadata logged correctly
- ✓ frame count + keyframe count + byte total accurate

Phase 8.4 next: wire daemon to /dev/daedalus-v4l2 chardev,
add REQ_DECODE / RESP_FRAME handling, drive VP9 decode
end-to-end via daedalus_dispatch_* from daedalus-fourier.

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

docs/phase_8_3_closure.md

@@ -0,0 +1,131 @@
+# Phase 8.3 closure — daemon with FFmpeg dlopen + parse path
+
+**Status:** closed 2026-05-18.
+
+Userspace daemon scaffold that loads FFmpeg via `dlopen` at
+runtime (Option γ, per locked Phase 8 architecture), opens
+container files via libavformat, and iterates per-frame
+metadata.  No V4L2 / chardev / decode work yet — pure
+parse-path validation.
+
+## What lands
+
+- `daemon/CMakeLists.txt` — CMake build; pkg-config for FFmpeg
+  headers; `-Wall -Wextra -Wpedantic` clean.
+- `daemon/src/main.c` — entry point, signal handlers, command
+  dispatcher.  Currently exposes `parse <file>`.
+- `daemon/src/ffmpeg_loader.{c,h}` — runtime FFmpeg loader.
+  dlopens `libavformat.so.61`, `libavcodec.so.61`,
+  `libavutil.so.59`; resolves the 22 function pointers we use.
+- `daemon/src/parser.{c,h}` — demux loop using
+  `avformat_open_input` + `av_read_frame`; logs per-frame
+  metadata.
+- `daemon/src/log.{c,h}` — minimal logging facade
+  (stderr-only Phase 8.3; syslog/journal in Phase 8.5+).
+
+## Build
+
+```sh
+cd ~/src/daedalus-v4l2/daemon
+mkdir build && cd build
+cmake ..
+cmake --build .
+```
+
+Build dependencies (Debian Trixie):
+`libavformat-dev libavcodec-dev libavutil-dev`.  Runtime
+dependencies are just `ffmpeg` (or rather: any package shipping
+the `.so.61`/`.so.59` files).
+
+## Verification
+
+```
+$ ffmpeg -hide_banner -f lavfi -i testsrc=duration=2:size=320x240:rate=30 \
+        -c:v libvpx-vp9 -y /tmp/testsrc.ivf
+$ ls /tmp/testsrc.ivf
+-rw-rw-r-- 1 mfritsche mfritsche 18611 May 18 17:09 /tmp/testsrc.ivf
+
+$ ~/src/daedalus-v4l2/daemon/build/daedalus_v4l2_daemon parse /tmp/testsrc.ivf
+[2026-05-18 17:09:54 INFO] FFmpeg loaded: 7.1.3-0+deb13u1+rpt1 (libavformat 61.7.100)
+[2026-05-18 17:09:54 INFO] video stream #0: codec=vp9 (Google VP9) 320x240, 0/0 fps
+[2026-05-18 17:09:54 INFO] parse complete: 60 frames (1 key) total 17859 bytes
+```
+
+60-frame VP9 file demuxed correctly; keyframe count and byte
+total match expectations.  `-v` flag enables per-frame DEBUG
+logs (size/pts/dts/flags).
+
+### Error paths
+
+```
+$ daedalus_v4l2_daemon parse /tmp/does_not_exist
+[INFO] FFmpeg loaded: ...
+[ERR ] avformat_open_input(/tmp/does_not_exist): code -2
+
+$ daedalus_v4l2_daemon
+usage: daedalus_v4l2_daemon <command> [args]
+...
+```
+
+Clean error reporting; exits with non-zero code.
+
+## Design notes
+
+### Why dlopen instead of static or dynamic-link?
+
+- **No version lock**: the daemon binary works against any
+  FFmpeg `.so.61` even if the patch version differs from build
+  time.  Future FFmpeg updates on the host don't force daemon
+  rebuilds.
+- **No transitive license issues**: linking dynamically against
+  FFmpeg (LGPL-2.1+) is fine for our BSD-2-Clause daemon, but
+  dlopen makes it explicit and inspection-friendly.
+- **Optional**: callers that don't need parsing (e.g., a
+  pre-parsed-bitstream test mode) can skip `ffmpeg_loader_init`
+  entirely.
+
+### Headers vs runtime
+
+We `#include <libavformat/avformat.h>` etc. at compile time for
+the struct + enum definitions (so our function-pointer
+signatures are type-safe).  We do NOT link against FFmpeg —
+the only loader dependency is `-ldl`.
+
+### POSIX dlsym to function pointer
+
+The dlsym → function pointer assignment uses the POSIX-
+recommended `*(void **) &fn_ptr = dlsym(...)` form to avoid
+the ISO C "function pointer is not a void pointer" warning.
+POSIX.1-2017 dlsym(3p) Rationale explicitly endorses this
+pattern.
+
+## What's NOT here
+
+- **Per-block metadata**: Phase 8.3 only demuxes containers
+  and logs frame-level info.  Per-block VP9/H.264 metadata
+  (block positions, MVs, coefficients) is Phase 8.4 work and
+  requires either FFmpeg internal APIs or our own bitstream
+  parsing.
+- **V4L2 / chardev wiring**: the daemon doesn't yet open
+  `/dev/daedalus-v4l2`.  Phase 8.4 connects the parse output
+  to the chardev request/response loop.
+- **Daemon mode**: no fork / setsid / pid file / systemd unit
+  yet.  Phase 8.5+ adds those when the daemon needs to run as
+  a long-running service.
+
+## Phase 8.4 plan
+
+1. Open `/dev/daedalus-v4l2` chardev.
+2. Wait for REQ_DECODE messages from kernel.
+3. For each request:
+   - Feed the bitstream blob into FFmpeg's parse path.
+   - Get an `AVFrame` back (FFmpeg decodes on CPU initially).
+   - Send RESP_FRAME with the decoded frame back.
+4. Validate end-to-end via a userspace V4L2 client that pushes
+   a VP9 file at the kernel module and reads decoded frames.
+
+The QPU dispatch via `daedalus_dispatch_*` from the sibling
+daedalus-fourier kernel library lands in Phase 8.4 as well —
+once the FFmpeg-driven decoder is working with CPU paths, we
+can substitute QPU dispatch for the kernels that benefit
+(cycles 1, 2, 4).