Port Skylands terrain filter (#47)

docs/AGE_PROFILES.md

@@ -56,7 +56,7 @@ 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
+random Age rolls zero, one, or two distinct compatible material 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
@@ -73,6 +73,24 @@ 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.
 
+**Skylands** is a separate subtractive feature, not the Sky Islands terrain
+profile or the additive Floating Islands feature. Following Legacy's primer
+filter, it removes base terrain below a smooth, seven-octave cutoff around
+world Y=76 and removes liquid above the cutoff. The selected terrain still
+generates first; Floating Islands and Tendrils are added afterward, so they
+can coexist with Skylands. Skylands has no Block-page modifier. It can be
+authored on every terrain, though Flat loses its low ground and Void has no
+base terrain to change. Page-free random noise Ages have a seeded one-in-eight
+chance to gain Skylands; random Flat and Void Ages do not roll this ineffective
+filter. The cloud-height and horizon changes from Legacy are not included yet.
+
+For an in-game check, compare Standard terrain with and without a Skylands
+page. Skylands should leave upper hills suspended over open space, while a
+Floating Islands page instead adds isolated high islands without removing the
+ground below. Try Sky Islands terrain plus Skylands to confirm the page filters
+the terrain rather than replacing its generator. A book with both Skylands and
+Floating Islands should retain the added feature islands above the cutoff.
+
 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-