Port Tendrils feature generation (#46)

Add the writable symbol, adapt Legacy's branching solid cave-node geometry across all terrain types, and test authored and random Age selection.

src/main/resources/assets/rivencraft/lang/en_us.json

@@ -245,6 +245,8 @@
   "symbol.rivencraft.feature.ravines.description": "Add vanilla canyon carving to noise terrain.",
   "symbol.rivencraft.feature.floating_islands": "Floating Islands",
   "symbol.rivencraft.feature.floating_islands.description": "Add high floating islands above the chosen surface terrain.",
+  "symbol.rivencraft.feature.tendrils": "Tendrils",
+  "symbol.rivencraft.feature.tendrils.description": "Add winding solid tendrils through the Age, using a random block or a preceding Block page.",
   "symbol.rivencraft.feature.lava_lakes": "Small Lakes",
   "symbol.rivencraft.feature.lava_lakes.description": "Add small surface lakes using a random fluid, or specify one with a fluid page.",
   "generator.rivencraft.standard": "Standard",

src/main/resources/data/rivencraft/rivencraft/symbols/feature/tendrils.json

@@ -0,0 +1,8 @@
+{
+  "schema_version": 2,
+  "name": "symbol.rivencraft.feature.tendrils",
+  "description": "symbol.rivencraft.feature.tendrils.description",
+  "category": "rivencraft:feature",
+  "semantics": { "role": "feature", "value": "rivencraft:tendrils" },
+  "poem": "Terrain Thread Structure Winding"
+}