Parallel tile decode gate excludes the default tile_size=256 #1551

Describe the bug

The parallel tile decode path in xrspatial/geotiff/_reader.py (around line 1091) is gated by:

use_parallel = (n_tiles > 1 and tile_pixels > 64 * 1024)

For the default tile_size=256, tile_pixels = 256 * 256 = 65536, which is exactly 64 * 1024. The strict > fails by one, so the sequential path runs even with 16+ tiles. The parallel decode only fires for tile_size > 256, but 256 is the default that most real GeoTIFFs ship with. The gate is closed on the most common case.

Expected behavior

The threshold should be >= so the default tile size engages parallel decode. The comment next to the line says the threshold is meant to "avoid pool overhead on small 64x64 / 128x128 tile reads" - 256x256 should clear it, not land on it.

Fix

One-character change:

use_parallel = (n_tiles > 1 and tile_pixels >= 64 * 1024)

Additional context

Found during an efficiency audit pass over the geotiff module on 2026-05-09.