perf(world_editor): single-traversal write on the default no-override path

[rzmo]

set_block(..., None, None) is the dominant write pattern across all element processors (~240 callsites). It currently goes through get_block() followed by set_block_with_properties(), descending the region/chunk/section FnvHashMap hierarchy twice per write.

This PR adds WorldToModify::set_with_props_if_absent, a single-traversal variant that does one entry().or_default() descent and checks section.storage.get(idx) == AIR directly. The None/None branch of set_block_with_properties_absolute routes to it via an early-return. The whitelist/blacklist slow path is untouched. Same pattern as the existing set_block_if_absent used by ground_generation.

SectionToModify::get_block returns None iff storage.get(idx) == AIR, so the condition checked is identical to the old path. Behaviour is preserved, only the number of map lookups is halved.

Benchmarked on Munich centre (48.125768,11.552296,48.148565,11.593838) with --terrain --benchmark --file over cached OSM data, release build, 5 runs each:

	Mean	Median
Before	9925 ms	9916 ms
After	9480 ms	9441 ms
Delta	-445 ms (-4.5%)	-475 ms (-4.8%)

All five after-runs were faster than all five before-runs.

[rzmo]

The benchmark ran successfully but the verdict was "generation time is unchanged" since CI rounds to whole seconds, so the local 4.5% improvement falls below CI's resolution. Can retrigger, I don't mind