Add Floating Islands and material-driven Age features (#45)

Keep repeated feature and material pages in authored writing, vary their resolved materials, and expose Link Panel modifier variants in creative and JEI. Remove Descriptive Book writing notes and refine Void biome selection.

docs/AGE_PROFILES.md

@@ -40,6 +40,39 @@ profiles, not claims that Legacy had identically named terrain symbols.
 Every family retains a safe cobblestone arrival platform. An omitted terrain
 page still uses the deterministic seed-selected profile.
 
+The separate **Floating Islands** feature page adds rare high-altitude island
+clusters above Standard, Amplified, Sky Islands, Flat, or Void terrain; it does
+not replace the terrain page. The feature is seeded by the Age and saved with the written
+plan, so revisiting an Age recreates the same islands. Without a preceding
+Block page, the Age seed chooses one valid full-collision block from the Block
+Pages pool for the island body. A full-block material page immediately before
+Floating Islands overrides that choice. Ordinary surface biomes still get
+dirt/grass top layers; Nether and End biomes use the chosen block throughout.
+The feature is additive even on Sky Islands terrain,
+which already forms its own islands. Cave terrain rejects it. A page-free random
+Age may also roll Floating Islands as one of its material features.
+
+An unpaired Block or Fluid page chooses a compatible feature from the Age seed:
+full blocks can become Rocks or Floating Islands; source fluids can become one
+of the lake features. Flat and Void terrain can only use Floating Islands from
+this pool, while Cave terrain has no compatible material feature. A page-free
+random Age rolls zero, one, or two distinct compatible features, choosing each
+material from the Block or Fluid Pages pool. The choices are saved on the Age
+definition, so opening the Age again does not reroll them.
+Repeated Block/Fluid and Feature pages remain separate entries in authored
+writing. Each unpaired material gets its own compatible feature, and repeated
+decorations use separate placement rolls. Repeated unmodified feature pages
+choose different compatible materials until their material pool is exhausted.
+
+For an in-game visual check, bind Descriptive Books with a Link Panel,
+one of Standard, Amplified, Sky Islands, Flat, or Void terrain, and the Floating Islands
+feature page in that order. Fly through each Age at roughly Y=170–220 and
+verify the elevated clusters appear above its unchanged base terrain. Reopen
+the same Age to check that island positions stay fixed. Islands should be less
+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.
+
 ## Written biome selection (generation version 16)
 
 Biome pages follow the terrain page and precede the distribution-control page
@@ -72,8 +105,15 @@ also accepted. Other cave/biome pairings are diagnosed and blocked for now.
 Milestone 6 instability should allow such illegal combinations as unstable
 Ages, without silently treating them as ordinary cave generation. Flat terrain
 uses Overworld choices when a distribution is authored; its Native default
-remains Plains. Void terrain defaults to The Void and needs additional authored
-biomes for a multi-biome distribution.
+remains Plains. Void terrain defaults to The Void when no biome page is written,
+but accepts authored biome pages. One authored biome uses Single distribution
+when no controller is written. An explicit multi-biome distribution needs enough
+authored biomes; The Void is not silently added to fill missing positions. These
+biome regions exist even without blocks or a Floating Islands page, so biome-
+dependent weather and optional season behavior can still vary there. To check
+this in-game, bind Void terrain with a Desert Biome page and no feature page:
+the Age should be empty while F3 reports Desert. A Void book without biome
+pages should instead report The Void.
 
 The resolved list of biome IDs, their order/repetitions, and distribution are
 frozen into the Age plan. A removed biome ID emits a warning and uses an