Revert to flexible flood-fill scanning with geometric chevron placement

The exact-template approach from the last commit over-fit to the
sample ASCII diagram, which was meant to teach the target shape, not
be matched literally cell-for-cell. Back to flood-filling any
connected, fully-enclosed ring the builder makes (any size/shape), but
chevrons are no longer evenly spaced by scan order - the scanner walks
the ring in geometric order and marks local maxima of distance from
the interior's centroid, i.e. the blocks that stick out further than
their ring-neighbours. That's where a real Stargate's chevrons sit
(the 11-wide reference ring naturally produces 7 this way: the top
apex, four shoulder elbows, two side bumps), without hardcoding a
specific size.

Falls back to the old evenly-spaced picker only if the ring isn't a
simple single-thickness loop (each frame block should have exactly
two in-plane neighbours) so odd/thick builds still work.

gate.max-frame-blocks/max-iris-blocks/min-frame-blocks are back;
gate.template is gone.

README.md

@@ -16,38 +16,21 @@ Build everything with `./gradlew build`. Jars land in each module's `build/libs/
 
 ## Building a gate
 
-A gate's shape is a fixed template in `config.yml` (`gate.template`), matched exactly
-against the world - not flood-filled or guessed. The default is an 11x11 ring with 7
-chevrons, laid out top row to bottom row:
+There's no fixed size or shape - build any single connected ring out of
+`gate.frame-materials` (default: obsidian, gold block, gilded blackstone, birch/oak
+planks) with a fully sealed hollow interior. No gaps; the interior must be enclosed.
 
-```
-00000000000
-0000*1*0000
-00012221000
-00*22222*00
-0*2222222*0
-01222222210
-0*2222222*0
-00*22222*00
-00012221000
-0000***0000
-00000000000
-```
+Chevrons aren't a separate material you place. When the gate is linked, the plugin
+flood-fills the ring, walks it in geometric order, and picks out the blocks that stick
+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
+invisibly into the frame until it lights up.
 
-- `*` — frame block, never touched by the plugin. Must be one of `gate.frame-materials`
-  (default: obsidian, gold block, gilded blackstone, birch/oak planks).
-- `1` — a chevron: a frame block (same material rules as `*`) that swaps to
-  `gate.chevron-lit-material` (default gilded blackstone) while dialing/open, then
-  reverts to whatever it 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.
-- `2` — interior ("iris"), filled with `gate.iris-material` (default water) while open.
-- `0` — ignored, not part of the gate.
-
-Build it in a single vertical plane, one block thick, aligned to the world's X or Z
-axis (facing north/south/east/west - not built at an angle). Edit `gate.template` in
-`config.yml` if you want a different size or chevron count/placement.
-
-1. Build the frame to match the template.
+1. Build the ring.
 2. 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]`
@@ -55,13 +38,12 @@ axis (facing north/south/east/west - not built at an angle). Edit `gate.template
    - 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`, stand
-   facing the gate square-on and left-click (punch) any `*` or `1` block, within
-   `gate.max-link-distance` of the sign. The plugin matches the template against the
-   world starting from that block and links the sign to it.
+3. 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.
 
 If punching a block doesn't work, the plugin tells you exactly why in chat - wrong
-material, no orientation of the template lines up, too far from the sign, etc.
+material, unsealed interior, too far from the sign, etc.
 
 ## Using a gate