v0.5.0 — terrain toolkit + server-side terrain tools wiring

Terrain support for the builder.

terrain toolkit (tools/terrain/) — the 2.5-D counterpart of the voxel toolkit: author a HeightField (multi-octave noise, radial falloff, blob lakes, carved rivers, build pads), erode it (hydraulic + thermal), render-verify offline (hillshade / relief / cross-section profile) before placing, then materialise to fills (double-layer substrate, no-monoculture surface mix, cliffs, beaches, water columns) and place via the shared mcp_place.py. numpy + Pillow only.

Skills wired to the mod's v0.3.0 terrain tools, fallback-gated — reference/engine-limits.md gains a "Terrain helpers" section; terraforming (landforms, command-budget, weathering, palettes, SKILL) and surveyor cite block_fill_columns, level_place_feature, level_fill_biome, the block_get_top_y heightmap arg, and the block_render_region hillshade view — each "prefer if available, else current approach" so it stays safe on older mods.

Pairs with minecraft-java-fabric-mcp-server v0.3.0.