Skip empty random-tick sections

[HabsW]

Moonrise already skips the inner random-tick work when a section has no ticking blocks, but the outer loop still walks every section sequentially.

This change keeps a lightweight per-chunk bitset of sections with tickingBlockCount > 0. Sparse chunks only visit sections with active ticking blocks (in the exact same section index order as before), while dense chunks retain the original linear scan. Because empty sections never consumed random-tick RNG, skipping them maintains identical game timing and behavior. The mask is built on load, updated on 0/non-zero transitions, and rebound on the next random tick if a plugin replaces a section object through getSections() (WorldEdit/FAWE).

Benchmarks (optimiseRandomTick isolated)

Note: These metrics measure duration inside optimiseRandomTick only, not overall server MSPT.

Measured across 1,024 forceloaded chunks (random_tick_speed=3) on a Ryzen 9 9950X3D / JDK 25:

• Normal Overworld: No change (613 µs → 614 µs)
• Superflat World: 422 µs → 387 µs (1.09x)
• Skyblock (Void + Grass): 231 µs → 121 µs (1.90x)
• The End (Main Island): 234 µs → 115 µs (2.04x)

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Reproduction & Reference

• Detailed Benchmarks: Machine, world settings, and equivalence results are documented in docs/TEST.md.
• Reference Prototype Patch: View the standalone Paper patch 0001-Skip-empty-random-tick-sections.patch.