core/teardown: ask pixelpass to stop before killing it

The peerspeak half of phase 0c (design v3.4 §7.4 item 1). Stop Share and
session teardown both went straight to `Child::kill()`, i.e. SIGKILL, which
skips pixelpass's own cleanup and leaks one null-sink module every time.

`ReapOnDrop::shutdown` now asks first: SIGINT, a bounded 2 s wait, then SIGKILL
only if the child ignored the request. SIGINT specifically, not SIGTERM —
pixelpass installs only a `ctrl_c()` handler, so SIGTERM would take the default
disposition and be indistinguishable from SIGKILL.

Signalling by pid is safe against pid reuse here: we have not reaped the child,
so it is a zombie whose pid the kernel reserves until we wait it, and the pid
cannot name a stranger. (Same reasoning that dismissed pixelpass bug #6.)

The grace is 2 s because it is awaited inline in the core command loop, so it
is also how long a wedged child can delay other commands. A healthy pixelpass
never spends it.

The drop/unwind path deliberately stays a hard kill: `Drop` cannot await, and
there the ordering invariant (§7.1) outranks tidiness. Once the pixelpass half
of 0c lands, the capture sink is connection-owned and that path stops leaking
by construction.

`libc` becomes a direct unix-only dependency, pinned to 0.2.186 — the version
already in the tree via alsa/cpal/tokio — so Cargo.lock gains one line and no
new code enters the build.

Mutation-verified, five mutations, each killing its own gate: no wait (6 fail),
reversed field order (2, reap test green), no reap loop (2, ordering test
green), no SIGINT (6), no SIGKILL fallback (exactly 1 — the wedged-child test).
633 lib tests, clippy clean, fmt clean.

Not yet field-tested: the live Stop Share gate (SIGINT sent, child exits within
the bound, no fallback kill on the normal path) still owes a real run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Cargo.toml

@@ -109,3 +109,10 @@ windows-sys = { version = "0.61", features = [
     "Win32_System_Diagnostics_ToolHelp",
     "Win32_System_Threading",
 ] }
+
+# Unix-only. Used for exactly one thing: sending SIGINT to our own
+# pixelpass child so it can run its cleanup before we resort to SIGKILL
+# (src/core/teardown.rs). Already in the tree via alsa/cpal/tokio, so
+# declaring it directly adds no new code to the build.
+[target.'cfg(unix)'.dependencies]
+libc = "0.2.186"