host/taint: owner key 4 falls back to the Client's pipewire.sec.pid

Native PipeWire clients put no application.process.id on their nodes —
only client.id. keys_of read node properties alone, so those nodes had no
key 4, were therefore unbounded, and propagate_unresolved_owner excluded
them the moment any tainted reader existed anywhere on the machine.

Measured: an untagged mpv was eligible alone, and became unresolved-owner
the instant peerspeak played audio. Since peerspeak playing audio is the
only situation in which this feature runs, that amounted to "native
PipeWire apps are never shareable". The tainted reader that armed it was
sunshine, which is itself bounded — so this is the bounded-reader arm,
not the keyless-reader case §6.1.1 narrates.

The pid is one hop away, on the node's Client, already in the snapshot.

RISK, and the guard on it: every Pulse-emulated Client carries
pipewire-pulse's own PID as sec_pid — measured, 15 unrelated Clients
sharing 2528 on this host. An unguarded fallback would fuse all of them
into one owner. Exception 1 therefore applies to the fallback exactly as
it does to the node's own property, so the fallback strictly *adds*
correct bounding rather than trading it.

Ambiguous client ids yield no fallback pid: inventing an owner key is the
one direction that can reduce taint, so a coin toss is the wrong guess.

The client index is threaded through a new OwnerCtx rather than a sixth
positional Option<u32>, and evaluate() builds one and shares it, so the
components and the key index cannot disagree about who is bounded.

Round 10, R10-3. 6 new rows; 3 mutations verified — removing the
fallback, dropping the pulse-pid exception (11 rows die), and resolving
an ambiguous client id instead of dropping it.

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

src/host/audit/tests.rs

@@ -150,7 +150,10 @@ fn an_ignored_ownership_tag_is_reported_without_excluding_anything() {
     let row = &body.ignored_ownership_tags[0];
     assert_eq!(row.serial, impostor.serial.0);
     assert_eq!(row.role, "stream-input");
-    assert_eq!(row.name.as_deref(), Some(owned_name("rogue", 4_242).as_str()));
+    assert_eq!(
+        row.name.as_deref(),
+        Some(owned_name("rogue", 4_242).as_str())
+    );
 }
 
 /// The common path stays quiet: a correctly tagged peerspeak producer is