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>

tests/fixtures/ownership-tag-contract.txt

@@ -0,0 +1,34 @@
+# Screenshare audio exclusion — ownership tagging wire contract.
+#
+# peerspeak PRODUCES these carriers on every audio node it owns; pixelpass
+# CONSUMES them as the primary taint root of the exclusion engine. Neither
+# repo depends on the other, so this file is the contract: it is committed
+# byte-identical in both, and each repo has a test that asserts its own named
+# constants (and, on the producer side, the environment a real child Command
+# would carry) match these values exactly.
+#
+#   peerspeak/tests/fixtures/ownership-tag-contract.txt
+#   pixelpass/tests/fixtures/ownership-tag-contract.txt
+#
+# Pinned by peerspeak docs/screenshare-audio-exclusion-impl-plan.md §3 and
+# docs/screenshare-audio-exclusion-plan.md §5.1 (v3.5). Changing a value here
+# is a cross-repo breaking change: both repos must land in the same session,
+# and the phase 5 matrix must be re-run.
+#
+# Two carriers, matched as a UNION — a node is peerspeak-owned if EITHER
+# matches. Round 8 added the second because a property is invisible to the
+# PipeWire registry `global` event and readable only via a node bind, so the
+# primary taint root must not rest on one observation mechanism alone.
+
+# Carrier 1 — a node property. The consumer treats any value other than
+# "false"/"0" as truthy, which is the fail-closed direction; the producer
+# always emits exactly this value.
+prop_key=peerspeak.owned
+prop_value=1
+
+# Carrier 2 — a `node.name` prefix, announced by the registry without a bind.
+# `node.description` is deliberately NOT touched, so mixers still show "mpv".
+# Only the prefix is matched; the rest of the name is for diagnostics.
+node_name_prefix=peerspeak_owned_
+node_name_format=peerspeak_owned_<role>_<pid>
+node_name_example=peerspeak_owned_mpv_31284