pack: KubeJS tidying + admin commands, bump to 0.20.0

Tidying pass on the KubeJS suite before plot system V1 lands. Sets up
the file structure + conventions + diagnostics so the larger plot
system has a clean foundation.

Changes:

1. NEW kubejs/README.md
   File layout doc + conventions: logging prefix [bns/<module>],
   persistent-data key naming (bns* prefix), one concern per file,
   performance discipline (O(1) lookups, no polling).

2. Split economy_policy.js -> recipes_waystones.js + recipes_numismatics.js
   Single-concern files. recipes_waystones removes ALL waystones
   crafting (welcome.js distributes the only allowed waystone instead).
   recipes_numismatics removes the 5 coin-denomination crafts.

3. NEW kubejs/server_scripts/bns_admin.js
   /bns admin subcommands for OP diagnostics:
     - info                       calling player's flags+counts
     - waystone-count <player>    read stored count
     - waystone-set <player> <n>  override stored count
     - reset-welcome <player>     clear bnsWelcomeGranted -- replays
                                  the welcome grant on next login
   Registered via ServerEvents.commandRegistry with Brigadier tree,
   permission level 2 (OP). No performance cost (operator-triggered only).

4. Logging added to waystones_policy.js + welcome.js
   Every action prints `[bns/<module>] ...` so server logs can be
   filtered with `journalctl ... | grep '\[bns/'`.

Performance discipline applied:
- All event-handler lookups use Set.has (O(1)), not Array.includes
- No periodic tick polling anywhere
- No network calls in event handlers
- console.info is cheap (no string format unless logged)

Bumps to 0.20.0. Plot system V1 follows in v0.21.x as a separate
multi-file submodule under server_scripts/plots/.

pack/overrides/kubejs/server_scripts/waystones_policy.js

@@ -62,6 +62,7 @@ BlockEvents.placed(event => {
             'That block is disabled. Use a regular Waystone (1 per player) ' +
             'and build transport between distant bases.'
         ));
+        console.info(`[bns/waystones_policy] denied ${blockId} placement by ${player.username} (banned)`);
         return;
     }
 
@@ -74,9 +75,11 @@ BlockEvents.placed(event => {
                 `You already have ${WAYSTONE_CAP} waystone. Break your existing one ` +
                 `to relocate, or build infrastructure to reach distant bases.`
             ));
+            console.info(`[bns/waystones_policy] denied ${blockId} placement by ${player.username} (already at cap ${current})`);
             return;
         }
         data.putInt('bnsWaystoneCount', current + 1);
+        console.info(`[bns/waystones_policy] +1 waystone for ${player.username} (now ${current + 1})`);
     }
 });
 
@@ -89,5 +92,6 @@ BlockEvents.broken(event => {
     const current = data.getInt('bnsWaystoneCount');
     if (current > 0) {
         data.putInt('bnsWaystoneCount', current - 1);
+        console.info(`[bns/waystones_policy] -1 waystone for ${player.username} (now ${current - 1})`);
     }
 });