feat(worldgen): growing cycle — longer Nether, alternating OW gaps, per-period doubling

[bh679]

What

Reworks the repeating worldgen cycle (OW → Nether → OW → Void → End islands → Void → repeat) so the journey escalates with distance:

1. Nether core 400 → 5000 — the real-Nether stretch is now a real destination, matching the End-islands core.
2. Asymmetric overworld gaps — 10000 before each Nether band (Void→Nether) and 5000 before each End band (Nether→Void), instead of one shared 10000.
3. Per-period doubling — new disintegrationPeriodGrowthFactor (default 2). Each successive period multiplies the scaled segments (Nether core, void hold, End-islands core, both OW gaps) by the factor; the transition fades stay fixed. 1 = classic uniform cycle.

How

• WorldGenCycle gains a period-aware locate() that walks the geometrically-growing periods (O(1) floorMod fast-path when growthFactor=1), with an int-safe clamp so scaled spans never overflow (growth is bounded by the world border anyway). All ramp methods evaluate against the per-period scaled spans; the pure NetherTransition/Disintegration ramp helpers are unchanged.
• Config: split the OW-gap key into before-Nether / before-End, raised the Nether-core default, added the growth-factor knob (all with dev-test presets where applicable). Every consumer routes through WorldGenCycle.fromConfig(), so no caller signatures change.

Mod-impact

Worldgen change. 2 new COMMON config keys + 1 raised default. No new deps, mixins, registries, or packets.

Testing

• ./gradlew build + full test suite — green (incl. merge with main fix(nether): collision-check ghast spawns so they don't suffocate in blocks #556–558).
• WorldGenCycleTest extended: new asymmetric-gap and per-period-doubling tests (assert period-1 segments are 2× period-0 at the accumulated offsets); existing tests migrated to the 14-arg ctor with growthFactor=1 (identical behaviour) and still pass.
• Manual (Gate 2): dev client launched clean; cycle layout confirmed in dev-test compact scale. Release 5k/10k figures verified via unit tests (dev builds auto-shrink by design).

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