audio: render only the frames PipeWire requests per cycle

The process callback filled the whole mapped buffer every cycle. PipeWire
sizes that buffer by clock.quantum-limit (8192 frames by default) while the
graph drives galdr at the 256-frame quantum we ask for, so each cycle did
~32x the work and everything past the quantum was dropped: choppy output,
constant deadline misses in pw-top, and envelopes that sounded 32x faster
than their settings. Use Buffer::requested (pipewire feature v0_3_49),
clamped to the buffer, with the --latency value as fallback.

tui: typing a glyph no longer advances the cursor to the right.

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

Cargo.toml

@@ -11,7 +11,7 @@ codegen-units = 1
 
 [dependencies]
 anyhow = "1"
-pipewire = "0.10"
+pipewire = { version = "0.10", features = ["v0_3_49"] } # Buffer::requested
 midir = "0.10"
 arc-swap = "1.7"
 rtrb = "0.3"

src/audio.rs

@@ -75,6 +75,7 @@ fn run(
         .add_local_listener_with_user_data(())
         .process(move |stream, _| {
             let Some(mut buffer) = stream.dequeue_buffer() else { return };
+            let requested = buffer.requested() as usize;
             let datas = buffer.datas_mut();
             let Some(data) = datas.first_mut() else { return };
             let stride = ch * 4;
@@ -85,9 +86,15 @@ fn run(
                 engine.apply(ev);
             }
 
+            // Render only what the graph asked for this cycle. The mapped
+            // buffer is sized by clock.quantum-limit (8192 frames by default),
+            // so filling it whole would do ~32x the work of a 256-frame
+            // quantum and PipeWire would drop everything past the quantum:
+            // choppy output and envelopes that race through their stages.
+            let want = if requested > 0 { requested } else { latency as usize };
             let mut n_frames = 0usize;
             if let Some(slice) = data.data() {
-                n_frames = slice.len() / stride;
+                n_frames = want.min(slice.len() / stride);
                 for f in 0..n_frames {
                     let smp = engine.render(&program);
                     let bytes = smp.to_le_bytes();

src/tui.rs

@@ -169,7 +169,6 @@ pub fn run(
                 KeyCode::Char(c) => {
                     grid.set(cx, cy, if c == ' ' { '.' } else { c });
                     edited = true;
-                    cx = (cx + 1).min(grid.w - 1);
                 }
                 _ => {}
             }