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.

docs/AGE_PROFILES.md

@@ -73,6 +73,23 @@ common than in the first test build (one source chunk in 128 instead of 64).
 Sky Islands should retain its terrain islands and gain occasional additional
 feature islands. Try Cave terrain too; the Binder should reject that pairing.
 
+The **Tendrils** feature adapts Legacy's winding, branching cave-node geometry
+into solid tubes. It is additive on all current terrain families, including
+Flat, Void, and Cave; it does not change biome selection. A preceding full-
+collision Block page chooses the tendril material. Otherwise the Age seed
+chooses a valid Block Pages material, consistent with other authored material
+features. Page-free random Ages can select Tendrils as a solid-material feature,
+including on Cave terrain where the surface material features are unavailable.
+The shape and placement use only the Age seed and source-chunk coordinates, so
+revisiting an Age recreates the same geometry.
+
+For a clear visual check, bind a Void Age with a Glowstone Block page followed
+by Tendrils. Fly around world Y=0–120 across several chunks: branching solid
+forms should appear in the otherwise empty dimension, continue across chunk
+borders, and remain in place after leaving and returning. Repeat with a lone
+Tendrils page to confirm its seed-chosen material, and with Cave terrain to
+confirm the feature does not require a surface biome.
+
 ## Written biome selection (generation version 16)
 
 Biome pages follow the terrain page and precede the distribution-control page

docs/LEGACY_AGE_GRAMMAR_AUDIT.md

@@ -18,7 +18,7 @@ Legacy does **not** reject a book just because it omits terrain, biome, weather,
 | `WeatherNorm`, `Fast`, `Slow`, `Off`, `On`, `Cloudy`, `Rain`, `Snow`, `Storm` | `AgeTraits.Weather` has matching names except Legacy `On` maps to `ALWAYS` | Compile weather semantics separately from biome selection. `Snow` must remain deferred until optional season integration; do not silently substitute another weather mode. |
 | Suns, moons, starfields; angle, phase, period, color/gradient/sunset modifiers | One `celestialPeriod`, `celestialAngle`, `celestialPhase` on `AgeTraits` | Compile only the representable single-cycle subset first. Multiple celestial bodies, independent periods, custom stars, gradients and sunsets need a richer definition and renderer, not flattening into one value. |
 | Sky/fog/cloud/grass/foliage/water colors, natural variants, horizon, rainbow | Existing sky/dimension visuals and preview | Require an explicit modern client/biome-color design; metadata-only symbol pages must not pretend to change rendering. |
-| Caves, ores, huge trees, lakes, floating islands, tendrils, skylands, spheres, spikes, crystals; vanilla villages, strongholds, mineshafts, dungeons, fortresses, obelisks, fissures | Current cave profile and vanilla data-pack generation | Compile only features for which registered 1.21.1 placed features/structures exist. Defer bespoke structures and polish to milestone 7. |
+| Caves, ores, huge trees, lakes, floating islands, tendrils, skylands, spheres, spikes, crystals; vanilla villages, strongholds, mineshafts, dungeons, fortresses, obelisks, fissures | Current cave profile, vanilla data-pack generation, and authored Floating Islands/Tendrils | Compile supported vanilla placements and port bespoke shapes explicitly. Remaining bespoke structures and polish stay in later issues. |
 | `EnvAccel`, explosions, lightning, meteors, scorching; PvP and dummy suppression pages | No equivalent Age-definition effect | Defer environmental hazards/instability effects to milestone 6, PvP rule to a separately scoped server policy; retain authored IDs without fake behavior. |
 
 The Legacy symbol list and page names come from [ModSymbols](https://github.com/Mystcraft/Mystcraft-Legacy/blob/9bc8ddc061845df0cd5ea47f4fdef773cff786ae/src/main/java/com/xcompwiz/mystcraft/data/ModSymbols.java#L129-L240) and [SymbolRules](https://github.com/Mystcraft/Mystcraft-Legacy/blob/9bc8ddc061845df0cd5ea47f4fdef773cff786ae/src/main/java/com/xcompwiz/mystcraft/data/SymbolRules.java#L19-L137). Rivencraft's [catalog](../src/main/java/net/dnorberg/rivencraft/writing/SymbolCatalog.java) currently holds display metadata only; its built-in set is just standard terrain, native biomes, normal weather, normal lighting, normal time, and caves. [OrderedWriting](../src/main/java/net/dnorberg/rivencraft/writing/OrderedWriting.java) preserves page order, but no compiler yet connects it to [AgeTraits](../src/main/java/net/dnorberg/rivencraft/book/AgeTraits.java) or [Age profiles](AGE_PROFILES.md).