Fix one-way travel, rework iris to SG-1 mechanics, glowstone chevrons, pin-locked shield

One-way travel bug: the exit point landed players directly inside the
destination gate's own iris footprint. Since dialing opens both ends,
arriving there immediately re-triggered the teleport listener and
bounced them straight back - reading exactly like one-way travel.
GateManager.computeSafeLanding now nudges the landing spot along the
player's travel direction until it clears the destination's iris
blocks, and GateTeleportListener adds a short per-player cooldown as a
second guard against re-trigger. Same-server teleports now also
preserve the player's exact yaw/pitch and velocity instead of forcing
a fixed stored orientation, so entering forward always means exiting
forward; the cross-server plugin-messaging protocol was extended to
carry yaw/pitch through the proxy hop for the same reason.

Iris rework to match SG-1 rather than a binary open/closed gate:
- Idle (not connected): iris-idle-material, default AIR - you see
  straight through an inactive ring, not a wall.
- Connected: iris-open-material (WATER, the event horizon) unless the
  gate's separate iris shield is closed, in which case
  iris-shield-material (IRON_BLOCK) blocks travel even though the
  wormhole is active.
- A button placed directly below the control sign toggles the shield
  (GateIrisButtonListener). Closing needs nothing; opening a gate with
  a pin code set (/sg pin <code>) prompts the player to type it in
  chat within gate.pin-timeout-seconds first.

Also: gate.chevron-lit-material default changed from gilded blackstone
to glowstone per feedback.

README.md

@@ -34,19 +34,22 @@ further outward than their immediate ring-neighbours - the "elbow" points of the
 shape, which is where a real Stargate's chevrons sit (an 11-wide octagonal ring like
 the reference design naturally produces 7 of them: the top apex, the four shoulder
 elbows, and the two side bumps). Those blocks swap to `gate.chevron-lit-material`
-(default gilded blackstone) while dialing/open, and revert to whatever they looked
-like at rest when the gate closes - so a chevron can rest as plain obsidian and blend
+(default glowstone) while dialing/open, and revert to whatever they looked like at
+rest when the gate closes - so a chevron can rest as plain obsidian and blend
 invisibly into the frame until it lights up.
 
 1. Build the ring.
-2. Place a sign anywhere - it doesn't need to touch the frame, so it can act like a DHD
+2. Optionally place a button directly below where you'll put the sign - this becomes
+   the iris shield toggle (see below). Not required; a gate without one just has no
+   shield control.
+3. Place a sign anywhere - it doesn't need to touch the frame, so it can act like a DHD
    console standing apart from the gate - with:
    - Line 1: `[Stargate]`
    - Line 2: network name (blank = default network)
    - Line 3: gate name (blank = auto-generated)
    - Line 4: `hidden` to keep it out of the cycle list, `fixed:GateName` to lock this
      gate to always dial `GateName` (no right-click cycling), or blank
-3. The sign will show "Punch the gate (30s)". Within `gate.link-timeout-seconds`,
+4. The sign will show "Punch the gate (30s)". Within `gate.link-timeout-seconds`,
    left-click (punch) any frame block, within `gate.max-link-distance` of the sign.
    The plugin scans out from that block and links the sign to it.
 
@@ -57,11 +60,21 @@ material, unsealed interior, too far from the sign, etc.
 
 - **Right-click** the sign: cycles the destination shown on line 3 among the other
   gates on the same network.
-- **Left-click** the sign: dials the shown destination — chevrons light in sequence,
-  then the interior switches from `gate.iris-closed-material` (default bedrock - solid,
-  nothing can pass through a closed/idle gate) to `gate.iris-open-material` (default
-  water, the "event horizon"). Walk into it to teleport. It auto-closes after
-  `dialing.open-seconds`, switching the interior back to bedrock.
+- **Left-click** the sign: dials the shown destination - chevrons light in sequence,
+  then the interior switches from `gate.iris-idle-material` (default air - an idle gate
+  is just an empty ring you can see straight through) to either the event horizon
+  (`gate.iris-open-material`, default water) or, if the iris shield is closed, to
+  `gate.iris-shield-material` (default iron block) instead, blocking travel even
+  though the gate is actively connected. It auto-closes after `dialing.open-seconds`,
+  switching back to the idle material either way.
+- Walking into the event horizon teleports you to the connected gate, arriving just
+  clear of its iris (never inside it - that would immediately teleport you right back)
+  and still facing whatever direction you were already walking, so entering forward
+  always means exiting forward.
+- **Right-click a button placed directly below the sign** toggles the iris shield.
+  Closing it never needs anything extra. If the gate has a pin set (`/sg pin <code>`),
+  opening a closed shield prompts you to type the code in chat within
+  `gate.pin-timeout-seconds` before it'll open.
 
 ## Multi-world
 
@@ -93,5 +106,8 @@ server's Stargate instance teleports them to the gate's exit point.
 ## Commands
 
 - `/sg list [network]`
+- `/sg networks`
 - `/sg destroy` (look at a gate's sign)
+- `/sg pin <code|clear>` (look at a gate's sign) — requires that code in chat before a
+  closed iris shield will open on that gate
 - `/sg reload`