feat(host): --no-mdns/PUNKTFUNK_MDNS gate + punktfunk1-host --pairing-pin — docker-testing enablers

mDNS advertisement (native _punktfunk._udp AND GameStream _nvstream) can now
be disabled with --no-mdns on serve/punktfunk1-host or PUNKTFUNK_MDNS=0
(the PUNKTFUNK_ZEROCOPY off-grammar) — for multicast-dead environments
(bridged Docker, CI netns) where the advert reaches nobody or fails outright
and aborts the GameStream plane; clients dial a manually-added host instead
(mDNS-blind host-add works since the 0.8.4 dial-first fix). Unit tests no
longer advertise on the LAN (mdns: false).

punktfunk1-host gains --pairing-pin <PIN> (the already-plumbed fixed-PIN
test seam, now CLI-exposed; empty refused like --mgmt-token) so a CI harness
runs a deterministic SPAKE2 ceremony instead of scraping the logged PIN.

Verified live: --no-mdns + PUNKTFUNK_MDNS=0 skip the advert, --pairing-pin
arms with the fixed PIN, and a punktfunk1-host --source synthetic +
punktfunk-probe loopback run passes verification (60/60 frames, 0 mismatch).

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

scripts/host.env.example

@@ -49,6 +49,8 @@ PUNKTFUNK_VIDEO_SOURCE=virtual
 #PUNKTFUNK_INPUT_BACKEND=libei      # wlr | libei | gamescope | uinput
 #PUNKTFUNK_FEC_PCT=20               # video FEC overhead percent
 #PUNKTFUNK_PERF=1                   # per-stage timing logs
+#PUNKTFUNK_MDNS=0                   # disable the mDNS adverts (native + GameStream) — for multicast-
+                                    # dead networks/containers; clients add the host by address instead
 # Display-management policy (keep-alive · topology · conflict · identity · layout · max) is set in the
 # web console (Host → Virtual displays) → ~/.config/punktfunk/display-settings.json, NOT here; a
 # settings file supersedes the legacy PUNKTFUNK_MONITOR_LINGER_MS / _NO_ISOLATE / *_VIRTUAL_PRIMARY