feat(hemp): wild patches in plains/forest + chest seed injection

Two new ways to find hemp seeds without already having any:

1. Worldgen: minecraft:scatter_feature spawns 1-3 mature
   silverlabs:hemp_crop blocks on grass/dirt in plains/forest/birch/
   flower_forest biomes (~14% scatter chance per chunk surface pass).
2. Chest injection: 8% chance per chest first-open to plant 1-3 seeds
   in a random empty slot. Tracked per-chest via world dynamic property
   (rolling cap of 500 entries) so each chest only contributes once.

Bedrock has no loot-table merge mechanism so we can't add seeds to
vanilla village chests without losing their vanilla loot — script
injection sidesteps that and stays version-independent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hemp-addon/hemp_BP/features/hemp_patch_feature.json

@@ -0,0 +1,12 @@
+{
+  "format_version": "1.21.0",
+  "minecraft:scatter_feature": {
+    "description": { "identifier": "silverlabs:hemp_patch_feature" },
+    "iterations": 3,
+    "scatter_chance": 65,
+    "x": { "distribution": "uniform", "extent": [-3, 4] },
+    "y": 0,
+    "z": { "distribution": "uniform", "extent": [-3, 4] },
+    "places_feature": "silverlabs:hemp_single_feature"
+  }
+}