feat(vdisplay): mode-conflict admission — separate/join/steal/reject (Stage 4)

The mode_conflict policy is now enforced at ADMISSION, before the punktfunk/1
Welcome, when a DIFFERENT client connects while another client's session is live:
- separate (default, unconfigured → no change): each client its own display.
- join: admit at the live display's mode (honest-downgrade — the Welcome carries it).
- steal: signal the victim session(s)' stop flags, wait the release grace, serve.
- reject: refuse the handshake with a busy reason (live mode + client label).

New vdisplay/admission.rs: the pure decide() (unit-tested — same-client never
conflicts, anonymous clients each distinct, join targets the oldest session) + a
live-session registry (identity + mode + stop flag) sessions register in once up.
Wired into punktfunk1 serve_session: admit() before validate_dimensions, register
after the data plane binds. A same-client reconnect never conflicts.

Validated on loopback (two probes, distinct identities, differing modes) across all
four policies: separate→own mode, join→live mode, steal→victim interrupted,
reject→handshake refused.

Remaining Stage-4 surface (deferred): GameStream 503 path, Windows-specific
defaults (separate→join map, silent-reconfigure→steal), reject reason delivered to
the client as a typed message (currently host-side log + connection close).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/vdisplay.rs

@@ -785,6 +785,10 @@ mod gamescope;
 #[allow(dead_code)]
 #[path = "vdisplay/identity.rs"]
 pub(crate) mod identity;
+// Platform-neutral mode-conflict admission (Stage 4): the separate/join/steal/reject decision + the
+// live-session registry, wired into the punktfunk/1 handshake.
+#[path = "vdisplay/admission.rs"]
+pub(crate) mod admission;
 #[cfg(target_os = "linux")]
 #[path = "vdisplay/linux/kwin.rs"]
 mod kwin;