8a: assert_no_alloc on audio-thread callbacks

Wraps the three audio-thread `process` callbacks
(`capture_process`, `playback_process`, `tap_process`) with
`assert_no_alloc::assert_no_alloc(|| inner(...))`. The
`headroom-cli` binary installs `AllocDisabler` as `#[global_allocator]`
so any allocation inside one of those blocks during debug builds
aborts the process with "memory allocation of N bytes failed".

Each callback was renamed to `*_inner` to keep the thin wrapper
function pointer stable for pipewire-rs's `process(fn_ptr)`.

`assert_no_alloc`'s `disable_release` is on by default — release
builds get the system allocator unwrapped and the macros become
no-ops, so the audio thread pays zero runtime cost in production.

Verified

  Positive smoke (5 s of 1 kHz sine through processed): daemon
  stays up across thousands of capture/playback/tap callbacks. No
  abort. Audio threads are alloc-free as designed.

  Negative smoke (temporarily inserted `Vec::with_capacity(1024)`
  inside `capture_process_inner`): daemon aborts (SIGABRT, exit
  134) on the first audio callback with the expected
  "memory allocation of 1024 bytes failed" stderr message —
  confirming the harness is wired correctly and not silently a
  no-op. Sanity-check alloc reverted before commit.

  185 tests pass; clippy clean at -D warnings --all-targets.

crates/headroom-cli/src/main.rs

@@ -14,6 +14,16 @@ use std::process::ExitCode;
 use clap::{Parser, Subcommand, ValueEnum};
 use headroom_client::{Client, ClientError, Route, Topic};
 
+// Wrap the system allocator so audio-thread `assert_no_alloc` blocks
+// in headroom-core can detect any allocation. In debug builds an
+// allocation inside such a block aborts the process — exactly what
+// we want when the daemon is exercised under `cargo run` or under
+// the test harness. In release builds the wrapper is a no-op
+// (assert_no_alloc's default `disable_release` feature), so there's
+// zero overhead in production.
+#[global_allocator]
+static ALLOCATOR: assert_no_alloc::AllocDisabler = assert_no_alloc::AllocDisabler;
+
 /// Headroom CLI.
 #[derive(Debug, Parser)]
 #[command(version, about, long_about = None)]