Port authored Age features and Star Fissures to 26.3 beta

Advance to 1.0.1.54-beta.1 after player validation. Port material, vegetation, lake, terrain-volume, crystal, Dense Ores and fissure features with saved plans. Update beta documentation and release notes.

Refs #98, #104

ports/mc263/RELEASE_NOTES_1.0.1.54-beta.1.md

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+# Rivencraft 26.3 beta 1.0.1.54-beta.1
+
+This beta collects the player-validated Age feature port slices since
+1.0.1.41-beta.1. It is not yet feature-complete with Minecraft 1.21.1.
+
+## Age generation
+
+- Rocks now use the established 1.21.1 forest-rock placement. Unmodified
+  material-bearing features choose seeded materials independently; a preceding
+  compatible Block or Fluid page selects the material explicitly.
+- Added Flowers, Trees, Huge Trees, Dungeons, Surface Lakes, Deep Lakes, and
+  Small Lakes using the 1.21.1 placement rules adapted to 26.3.
+- Added Floating Islands, Tendrils, Skylands, Spheres, Spikes, and Obelisks
+  using the existing Rivencraft generation algorithms.
+- Added blue Crystal Formations with budding crystal and initially seeded buds.
+- Added Dense Ores: repeated pages replay eligible biome ore placements,
+  including compatible modded ores and explicitly tagged custom placements.
+- Added Star Fissures: one Legacy-shaped rift near the first arrival point,
+  a starfield at the dimension's minimum Y, and return travel to the shared
+  Overworld spawn with the original fissure sound. Cave roofs are preserved.
+- Feature plans, materials, repeated occurrences, and fissure completion
+  persist across restart. Existing generated chunks are not redecorated.
+
+## Current limits
+
+- Authored features retain terrain restrictions. Most repeated features allow
+  up to 32 occurrences; exceeding the current authoring cap blocks assembly.
+  Skylands and Star Fissures retain their single-effect behavior when repeated.
+- Put a material page immediately before a compatible feature. Dangling
+  material assignment, full Legacy grammar recovery, instability, and random
+  feature selection are not yet ported. Pageless books randomize terrain and
+  biome distribution, not the newly authored feature list.
+- 26.3 combines base terrain, surface, and carving. Added solid volumes run
+  after that combined operation and before decoration, so base carvers do not
+  cut those added volumes.
+- The full 1.21.1 writing/discovery loop, structures, environmental systems,
+  crystal portals, and optional integration parity remain in progress. The
+  presence of a Creative page is not a promise that its effect is ported.
+
+Build and static checks passed. The player reported in-game passes for these
+slices. Earlier server smoke coverage was expanded, but the later generation
+slices were manually validated; this release does not claim a new full
+dedicated-server regression pass.
+
+Requires Minecraft **26.3**, NeoForge **26.3.0.99** or a compatible 26.3 build,
+and **Java 25**. Dynamic Dimensions and Bad Packets are not required by this
+26.3 build. Do not use Minecraft 1.21.1 dependencies or saves with it. Back up
+the entire world before updating; beta-save compatibility is not guaranteed.
+
+Report issues and help with documentation at
+[BlackSheep Git](https://git.paddedcell.org/dnorberg/rivencraft).