feat(space): mini-asteroids roll seeded sizes and mineral families, water ice included

[marceld23]

Summary

Every mineable space rock was an identical clone: fixed r=2 voxel sphere, titanium core, iron/copper/stone shell, hardcoded loot. Rocks now roll seeded mineral families and sizes — including water-ice asteroids.

Closes #687. Builds on #686 (EVA mining hardness parity).

Changes

Server (GameServerSpaceStructure.cs, GameServerSpaceCombat.cs)

• Families (weights): stony 40 % (iron veins in rock), metallic 25 % (the classic titanium-core rock), icy 20 % (hand-mineable water ice around a rocky heart), carbonaceous 10 %, crystalline 5 % — mirroring the landable asteroid families (Landable asteroids: every asteroid is the same crystal world — add asteroid families #515). All from existing blocks; zero new content.
• Sizes: r=1 pebble (7 blocks, common) / r=2 classic (33) / r=3 boulder (123, rare); the core grows with the rock. Hull tracking, laser carving and the client renderer were already size-agnostic — no client change.
• Shoot-down loot now matches family and size (boulders pay out more).
• Pinned anchor: rock 0 of every field stays the classic metallic r=2, so each field guarantees one titanium core (mirrors the start-planet ring pin) and existing mining tests keep their target.
• Determinism: a small xorshift seeded from WorldGenerator.StableHash(instance.Id) + world seed + spawn ordinal — integer state only: no Random, no string.GetHashCode (process-randomized), no trig-derived floats (Windows/Linux libm differ). Respawned rocks continue the ordinal sequence (3 + rotor).

Balance

With #686 in place the mining feel scales by material automatically: ice (hardness 0.8) pops in one bare-hand hit — an easy water harvest — while titanium keeps its tier-2 drill gate.

Tests

• New SpaceAsteroids_RollSeededFamiliesAndSizes: pinned metallic anchor, legal sphere sizes, same world → identical rocks (restart-safe), different seeds → actual variety.
• New test hook StructureCellForTest(structureId, x, y, z).
• Local: full Release build 0 warnings, 1364 server tests green.

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