fix(audio): honor per-cycle quantum in playback (A1, crackle on non-1024 hw)

The playback RT callback pinned the PipeWire buffer to exactly one 1024-frame
quantum (the 2026-05-31 crackle fix). That is only correct when the machine's
clock.quantum is 1024 — on hardware running quantum 512 or 2048 the pinned
slice mismatches the device's per-cycle demand and the crackle returns. We just
shipped a release to a friend whose quantum is unknown, so this was P1.

Fix: enable the pipewire `v0_3_49` feature (exposes Buffer::requested(), the
graph's per-cycle quantum) and fill exactly that many frames each callback via a
new pure `frames_to_produce()` seam, with a safe ≤1024 fallback when the graph
reports 0 (never the whole slice — over-pulling past the ring depth is the
original crackle). Relax the Buffers size pin from a hard 1024 to a generous
8192-frame max so the mapped slice fits any plausible quantum; requested(), not
the buffer size, now governs per-cycle output.

Verified locally with `pw-metadata clock.force-quantum` + audio_probe at forced
quanta 512/1024/2048: each shows `underrun +0` steady, `quantum=` matching the
forced value, and callbacks/s ≈ rate/quantum — proving requested() is live (the
health line would otherwise read the 1024 fallback). +4 unit tests on
frames_to_produce (148 lib tests, clippy --all-targets clean).

Still pending (field test): one real desktop<->dopedart call through the fix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -25,7 +25,11 @@ iced = { version = "0.14.0", features = ["canvas"] }
 iroh = "1.0.0-rc.0"
 iroh-gossip = "0.99.0"
 opus = "0.3.1"
-pipewire = "0.9"
+# v0_3_49 exposes `Buffer::requested()` (the graph's per-cycle quantum), used by
+# the playback RT callback to fill exactly what the device asks for instead of
+# pinning the buffer to a hard-coded 1024-frame quantum (crackle on non-1024
+# hardware). The field has existed in libpipewire since 0.3.49 (2022).
+pipewire = { version = "0.9", features = ["v0_3_49"] }
 rand = "0.10.1"
 ringbuf = "0.5.0"
 serde = { version = "1.0.228", features = ["derive"] }