Map Star Fissures to modern world minimum Y (#58)

docs/WRITING_GRAMMAR.md

@@ -20,6 +20,6 @@ Material pages are generated for registered full-collision, non-fluid blocks and
 
 Crystal Formations are an authored, material-free feature on exposed solid terrain, including Flat terrain; Cave and Void reject the page. Like Legacy, one in fifteen eligible chunks attempts a formation, with the site fixed by the Age seed and chunk coordinates. The modern formation is a compact mound with budding hearts instead of Legacy's long crystal lines. Budding blocks grow through small, medium, and large buds into clusters using vanilla amethyst timing and sounds. Non-silk cluster harvest yields Crystal Shards; four shards craft one Crystal Block. These blocks and shards use 16×16 textures: Legacy's crystal tile for solids and hue-shifted vanilla amethyst sprites for buds and clusters. A page-free Age can also randomly select Crystal Formations; an authored page always retains its exact choice in the saved plan.
 
-Star Fissures are a material-free feature on every terrain profile, including Cave and Void. The fissure is generated once beside the first resolved spawn, using Legacy's spawn-chunk offset and 10–17 paired, irregular X-span rows running along +Z. Each portal tile is placed at y=0 and its column is opened to the sky, as in Legacy. For modern terrain with a surface below y=0, the portal uses the minimum build height plus one instead; empty Void columns keep y=0. Its non-solid portal links to the Overworld spawn with the Legacy fissure sound; the animated starfield is drawn with Minecraft's current End Portal renderer, which uses the same end-sky/end-portal art as Legacy's custom renderer. Random Ages sample Legacy's default small-feature grammar for fissure selection: the stop/one/repeat branches have rank-derived weights 1:1:3, and the fissure has weight 1 of 9 among its small-feature rules. The remainder of random feature selection still follows Rivencraft's current pipeline, rather than running Legacy's entire Age grammar.
+Star Fissures are a material-free feature on every terrain profile, including Cave and Void. The fissure is generated once beside the first resolved spawn, using Legacy's spawn-chunk offset and 10–17 paired, irregular X-span rows running along +Z. Legacy's y=0 portal floor maps to the modern dimension's minimum build height, replacing lower bedrock if present. The rift opens to the sky on ordinary terrain; in Cave terrain it opens into the cavern but stops below the upper bedrock roof. Water in the carved columns is cleared before the portal is placed. Its non-solid portal links to the Overworld spawn with the Legacy fissure sound; the animated starfield is drawn with Minecraft's current End Portal renderer, which uses the same end-sky/end-portal art as Legacy's custom renderer. Random Ages sample Legacy's default small-feature grammar for fissure selection: the stop/one/repeat branches have rank-derived weights 1:1:3, and the fissure has weight 1 of 9 among its small-feature rules. The remainder of random feature selection still follows Rivencraft's current pipeline, rather than running Legacy's entire Age grammar.
 
 The Rivencraft symbol-page creative tabs read a generated index of bundled symbol definitions and group Pages by each definition's category, except biome-controller roles, which appear in a separate Biome Generation tab rather than the Biome tab. Adding another valid JSON symbol under `src/main/resources/data/rivencraft/rivencraft/symbols/` includes its page in the matching category tab on the next build; a new bundled category receives its own tab. Registry-derived Block and Fluid Pages have separate tabs and both appear in optional JEI. Link Pages have a separate tab. Blank Pages appear in neither the creative tabs nor JEI's ingredient list. The server catalog still decides whether a page is valid for writing. External server data-pack symbols are not included in this bundled index.