stems: multitrack PipeWire recorder

Records every source feeding the default sink into sample-aligned float WAV
stems plus the sink mix, so sessions can be mixed and mastered later.
Release flow mirrors galdr: tag -> build tarballs on both arches -> Gitea
release -> publish to the uhhm Arch registry (no AUR package).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

README.md

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+# stems
+
+Multitrack recorder for PipeWire. Run it, play your session, press Ctrl-C:
+every source that fed the default sink has its own WAV stem, plus the sink's
+mix as `00-master.wav`, all the same length and sample-aligned. Drop the
+folder into any DAW and mix or master later.
+
+```
+pw-play / galdr / browser / mic loopback ──▶ default sink ──▶ speakers
+          │            │           │              │
+          └── stems taps each source port ────────┴── 00-master (sink monitor)
+```
+
+## Usage
+
+```
+stems [--dir <path>] [--sink <node.name>] [--no-master] [--list]
+```
+
+- `--dir` session root, default `~/Recordings/stems`. Each run writes
+  `<dir>/<YYYY-MM-DD_HH-MM-SS>/NN-<source>.wav`.
+- `--sink` record the sources of this sink instead of the default one.
+- `--no-master` skip `00-master.wav`.
+- `--list` print the sources that would be recorded right now and exit.
+
+Files are 32-bit float WAV at the graph rate, one per source node, with the
+source's channel count (mono sources stay mono). Sources that start mid-session
+get a track the moment they link to the sink; sources that stop keep their
+file. Ctrl-C (or SIGTERM) finalizes everything and prints a table:
+
+```
+#   track                                     ch     length  starts at  dropped
+00  00-master.wav                              2      42.67s      0.04s        0
+01  01-galdr.wav                               2      42.67s      0.00s        0
+02  02-fc.zoom.playback.wav                    2      42.67s      0.00s        0
+03  03-pw-play.wav                             2      42.67s     12.30s        0
+```
+
+## How alignment works
+
+PipeWire stamps every graph cycle with one timestamp that all streams in that
+cycle see (`pw_time.now`). Each stem's first buffer is placed at that
+timestamp's distance from the session origin, cycles a stream missed are
+filled with silence, and at the end every file is padded to the common length.
+So frame N in one file is frame N in every other file; "starts at" in the
+table is just how much leading silence a late-joining track got.
+
+Each track is a capture stream linked by hand (`link-factory`) to the source's
+output ports, so the session manager never re-routes it, and the realtime
+callback only copies frames into a lock-free ring that a writer thread drains.
+
+## Limitations
+
+- A source that appears mid-session is tapped a few graph cycles after it
+  starts (the link has to be created once the graph reports it), so roughly
+  the first 30 to 50 ms of a new source are only in the master, not in its stem.
+- The default sink is read once at start; switching sinks during a session
+  is not followed. Use `--sink` to pin one.
+- Stems capture what the source sends to the sink, pre-fader: the sink's
+  per-stream volume is not applied (the master has it).
+
+## Build
+
+Rust, PipeWire 1.x. `cargo build --release`; the binary is `stems`.