Engage parallel tile decode for default tile_size=256

[brendancol]

Summary

Fixes #1551. Changes tile_pixels > 64 * 1024 to tile_pixels >= 64 * 1024 in _read_tiles, so the default tile_size=256 (256 * 256 = 65536 = exactly 64 * 1024) lands on the parallel decode path instead of falling one pixel short.

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

Benchmark

1024x1024 raster at default tile_size=256 (16 tiles), DEFLATE-compressed:

path	time
sequential (old, strict >)	15.7 ms
parallel (new, >=)	4.4 ms
speedup	3.56x

Larger grids scale further: 4.3x at 2048x2048, 5.3x at 4096x4096.

Test plan

• New test_parallel_decode_default_tile_1551.py with three cases:
  • parallel engages at default tile_size=256 with multiple tiles
  • sequential stays for small (128x128) tiles below threshold
  • sequential stays for single-tile reads regardless of size
• Full geotiff suite: 939 passed, 0 new failures (3 pre-existing matplotlib/Python 3.14 deepcopy failures in test_features.py::TestPalette are unrelated and reproduce on main)
• Tests use monkeypatch.setattr on concurrent.futures.ThreadPoolExecutor with no shared state, so pytest-xdist runs are safe

[Copilot]

Pull request overview

This PR fixes the parallel tile decode dispatch threshold in the GeoTIFF CPU reader so the default tile_size=256 (exactly 64 * 1024 pixels) uses the parallel decode path, addressing issue #1551 and improving decode performance for common tiled GeoTIFFs.

Changes:

• Change _read_tiles parallel-dispatch threshold from tile_pixels > 64 * 1024 to tile_pixels >= 64 * 1024.
• Add regression tests asserting parallel dispatch engages at tile_size=256, and remains sequential below threshold or when only a single tile job exists.
• Expand the inline comment in _read_tiles to document the inclusive boundary and the rationale (issue #1551).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
xrspatial/geotiff/_reader.py	Adjusts the parallel decode gating condition to include the default 256×256 tile size and documents the rationale.
xrspatial/geotiff/tests/test_parallel_decode_default_tile_1551.py	Adds regression coverage verifying dispatch behavior at the threshold boundary and for sequential-only cases.

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