feat(worldgen): beaches along seas and larger lakes (#679)

[marceld23]

Summary

Implements worldgen beaches (#679): sand (or a per-planet beachBlock) along the waterline of the sea and of larger lakes/ponds — on beach-masked stretches of coast, with a sandy shallow-water apron, palm-fringed tropical shores, and none on dry/airless/lava-sea worlds.

How it works

• Shore detection: a column is beach when it sits in a jittered 1..3-block band above the waterline AND real water lies within a probe ring (8 directions × radii 4/8/12, early-out) — inland lowland at coastal altitude never sand-coats. The submerged apron (seabed ≤3 below the sea line) needs no probe.
• Coast character: a large-scale FBM mask (~55–60 % beach) alternates sand with bare rocky shore; a small jitter noise makes the beach edge wander. Width follows slope automatically (flat coast → wide beach, cliff → sliver).
• Larger lakes: RiverField labels each pooled reach's fill-and-spill lake (connected coarse cells sharing one filled level) and pre-rings lakes with ≥64 visible water columns with dry shore markers (TryGetLakeShore). Small pools, 1-wide rivers and puddle ponds get no beach; large ponds (bowl ≥3 deep nearby) get a rim at the pond mask's edge.
• Material: new optional planet data key beachBlock (default sand, validated at content load). Beaches form only on WATER shorelines — lava seas keep their volcanic coasts. Surface AND sub-surface turn to the beach block, so the varied topsoil depth yields a real 2–5-block sand layer.
• Override order: biome → beach → snow/ice → volcano basalt — cold coasts get snow-dusted shores, volcano flanks still win.
• Consumers stay consistent: the shared IsBeachColumn query keeps Generate, tree stamping and tests agreeing. Beach columns grow only palms/dead snags (themes with neither leave the beach bare); giant fungi skip beaches; surface flora comes from the sand host pool at ×0.35 density.

Compatibility & performance

• Pure surface-palette change, all pieces are pure functions of (seed, x, z): no protocol or save changes, server and client preview agree. Existing worlds retroactively gain sandy shores (worldgen is seed-derived); player-built blocks are untouched.
• Per-column cost in Generate is O(1) gates; the probe runs only for columns inside the narrow waterline band. The lake-shore pass runs once per world inside the cached RiverField build and only touches lake-edge columns (~perimeter, not area).

Tests

5 new tests in BeachGenerationTests: sea coast grows beaches + Generate paints them, sandy apron in the shallows, no beaches on dry/lava/airless worlds, cross-instance determinism, and a synthetic-basin lake-shore ring incl. size threshold. Full non-Slow suite 1348/1348 and all 38 worldgen/river tests (incl. Slow) green locally; clean rebuild with 0 warnings.

Closes #679

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