feat(worldgen): asteroid belts — shared orbit annuli, belt mining density & ring-colour families

[marceld23]

Summary

Real asteroid belts (#683) + seeded planetary-ring colour families (#684).

Worldgen (new worlds only): all landable asteroids of a system now share 1–2 orbit annuli instead of scattering uniformly across the disc (which regularly parked them inside planet orbit lanes). The outer belt sits one orbit step beyond the outermost planet; big systems (5+ planets, 4+ asteroids) may roll a second, inner belt into a ≥620-unit gap between two planet orbits. Members occupy evenly spaced angular slots with a small wobble, so the flight-view clear-gap guarantee holds by construction. Geometry comes from new Hash01 salts (8xx series), never the body rng — and everything is gated on a new WorldDescription.AsteroidBelts flag (defaults false; ServerConfig's creation-time description carries true, mirroring SystemVariance; --belts off escape hatch), so existing universes regenerate byte-identically.

Mining density:

• S1: launching from an asteroid body spawns a dense 9-rock local field instead of the classic trio (per-instance respawn target).
• S2: every other asteroid body in the resident system carries a 4-rock mineable cluster at its flight-view position (server and client agree via the shared SystemBodyLayout.FlightViewScale; total capped at 24). Flying into the belt now means flying through rocks worth mining.
• Fix along the way: a never-launched ship's CurrentLocationId holds the default planet TYPE ("varied"), not a body id — the space-instance anchor now falls back to the save's active location for the asteroid logic. The hostile-archetype lookup deliberately keeps the legacy raw-id behaviour so fresh-start difficulty in pirate-space starts is unchanged.

Flight chart: belt members are grouped by orbit radius and drawn as ONE translucent band with a localized label (ui.map.belt: "Asteroid belt" / "Asteroidengürtel") instead of N stacked orbit rings; legacy scattered worlds keep per-body rings.

Ring colours (#684): PlanetRings.TintFor now rolls a seeded material family per ring system — ice white (55 %), dusty tan (20 %), rocky grey (15 %), rare pale violet (10 %) — consistent across flight view, surface sky and horizon band. Retroactive-safe: same RingSeed → same tilt/bands, only richer tints.

Tests

• New: UniverseTests.Belts_AsteroidsShareOneOrTwoAnnuli_ClearOfPlanetOrbits (annulus clustering, ≥200-unit clearance from every planet orbit, second-belt coverage), Belts_LeavePlanetsAndMoonsExactlyWhereTheLegacyLayoutPutThem, SpaceCombatTests.EnterSpace_ParksMineableClustersAtTheSystemsAsteroidBodies, EnterSpace_AtAnAsteroidBody_SpawnsADenseLocalField.
• Extended: SystemBodyLayoutTests + SystemChartLayoutTests sweeps now include a belt-enabled pass; star-centring zoom budget raised 2.0→2.1 (measured ~2.01× with the outer belt, documented in the test).
• Full non-Slow suites green locally: 1360 server + 147 client, 0 warnings (Release). Local Unity build succeeds.

closes #683
closes #684

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