memory-fix - #793

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When flush_region() saves a region to disk and removes it from memory,
subsequent block writes (e.g. tree canopy leaves crossing a region
boundary) would silently re-create the region via entry().or_default().
The re-created region contained only the stray blocks, and save() would
then overwrite the correct .mca file with this near-empty one — causing
entire regions to appear missing in the generated world.

Fix: track flushed region coordinates in a HashSet and skip all write
operations (set_block, set_block_with_properties, set_block_if_absent,
fill_column) targeting already-flushed regions. The skipped writes are
cosmetic (tree leaves at region edges) so there is no visual impact.

Closes the "partially empty world" bug introduced by the incremental
region flushing in PR #793.

…rites

Two fixes for the incremental region flushing introduced in PR #793:

1. Delay flush by one chunk past region boundaries so tree canopies
   (up to 3 blocks spill radius) are fully written before serialization.

2. Track flushed regions and purge any stale re-created entries at save
   time, preventing near-empty regions from overwriting correct .mca files.

No hot-path changes — the tracking is a single HashSet insert per flush,
and purge runs once at save time.

The incremental flush_region approach serialized regions sequentially
during ground generation instead of in parallel at save time. Benchmarks
show this causes a consistent ~47% time regression (30s → 44s) while
actually INCREASING peak memory (~935 MB → ~1060 MB) due to
serialization buffer overhead.

The memory reduction only helps for extremely large areas where in-memory
region data alone exceeds available RAM, but for those areas the time
penalty is proportionally worse (linear scaling).

This reverts the flush_region calls and all associated bookkeeping
(flushed_regions tracking, purge_stale_regions, delayed flush). The
flush_region method itself is removed as dead code. Regions are kept
in memory during generation and saved in parallel via rayon in
save_java(), restoring the original performance characteristics.