Decouple the control sign from the gate frame: place then punch to link

Sign creation no longer scans the block it's mounted on. Instead,
placing a "[Stargate]" sign starts a pending link (shown as "Punch the
gate" on the sign) and the next block the owner left-clicks within
gate.link-timeout-seconds and gate.max-link-distance becomes the scan
seed. This lets the sign act like a DHD console standing apart from
the gate instead of being physically attached to the frame.

- Gate now stores the punched link block's coordinates (link_x/y/z)
  so restarts re-scan from there instead of deriving a seed from the
  sign's attachment, which no longer applies.
- New GateLinkListener handles the punch and reports scan failures
  via the existing diagnostic messages; also clears pending state on
  disconnect.
- Exit-location facing is now derived from the punched frame block's
  position relative to the iris, since a WallSign facing is no longer
  guaranteed to exist or be relevant.

stargate-paper/src/main/resources/config.yml

@@ -46,6 +46,12 @@ gate:
   max-frame-blocks: 300
   max-iris-blocks: 200
   min-frame-blocks: 8
+  # After placing a "[Stargate]" sign, the owner has this long to left-click (punch) a
+  # block of the actual gate frame to link it - the sign itself can sit anywhere, like a
+  # DHD console away from the gate, instead of being mounted directly on the frame.
+  link-timeout-seconds: 30
+  # How far (in blocks) the punched frame block may be from the sign.
+  max-link-distance: 64
 
 dialing:
   # Seconds the gate stays open (iris filled) before auto-closing if nobody walks through.