phase 1: tag every audio node peerspeak owns, on both carriers

Zero behaviour change. This is what makes the screenshare exclusion
engine able to see us at all (plan §5.1, impl plan §3): pixelpass must
refuse to fan out our own playback, and until now it had no way to
recognise it.

Two carriers, matched by pixelpass as a union — `peerspeak.owned=1`
and a `node.name` prefix `peerspeak_owned_<role>_<pid>`. Round 8 added
the second after the phase-5 audit found a node property is invisible
to the PipeWire registry `global` event and recoverable only by
binding the node; the prefix is announced directly. A union is also
the fail-closed direction: a missed tag leaks call audio into a share,
a spurious one only over-excludes.

Three tagging sites, all three verified live on this host:
  - native call playback  → props on the stream dict
  - screenshare mpv/VLC   → PULSE_PROP + PIPEWIRE_PROPS on the child
  - notification chimes   → same, on pw-play/paplay

The literals are a cross-repo wire contract, so they appear once here
as named constants and are pinned in a fixture committed byte-identical
in both repos (tests/fixtures/ownership-tag-contract.txt). The contract
test is black-box: it builds a real child `Command` and reads back the
environment it would carry, rather than testing our own formatter.

Three live `#[ignore]`d exit-gate tests drive the real call sites and
poll `pw-dump` for the resulting node — the plan requires the tag be
shown landing on a live node, not just in the env. All three
mutation-verified (drop either carrier, or the role, and the matching
gate fails).

Measured while verifying: mpv, VLC, pw-play and paplay all honour
`node.name` from those env vars. The native stream set neither
`application.name` nor a description, so a mixer fell back to
`node.name` — which the tag turns into an internal identifier. Added
an explicit `node.description = "PeerSpeak"` there, which keeps the
plan's rule (the prefix must not reach `node.description`) while
preserving its intent: mixers stay readable.

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

src/audio/mod.rs

@@ -65,6 +65,10 @@ pub mod eq;
 pub mod gate;
 pub mod limiter;
 pub mod multitrack;
+// The cross-repo ownership tag (plan §5.1). Platform-neutral on purpose: the
+// carriers only matter on PipeWire, but the literals are a wire contract and
+// their test must run on every platform so a rename can't pass CI elsewhere.
+pub mod ownership;
 pub mod pan;
 // Linear resamplers used by the Windows/cpal backend (W4). Platform-neutral and
 // pure, so it builds (and its tests run) everywhere even though only the cpal