Add GigaPath-Flash and repair HIPT downloads

[CraigMyles]

Summary

• Add gigapath_flash, the pinned 384-dimensional tile encoder, and
  gigapath_flash_slide, its matched 384-dimensional LongNet slide encoder.
• Record the exact weight revisions and SHA-256 digests, preprocessing, constructor,
  and pinned upstream loader commit in output fingerprints.
• Make both GigaPath variants default to the released whole-slide contract:
  non-overlapping 256-pixel tiles on a 256-pixel lattice, with each tile centre-cropped
  to the encoder's 224-pixel input. Pass stored level-0 coordinates unchanged and set
  LongNet's runtime positional divisor to each tile's level-0 extent, normalizing
  raw2features' coordinate units into the tile-index frame of upstream's default
  256-pixel tiling.
• Existing original-GigaPath grids remain readable. The corrected default uses a
  distinct 256-pixel geometry key rather than aliasing an older 224-pixel grid.
• Repoint HIPT to Mahmood Lab's official commit-pinned Git LFS checkpoint. The checkpoint
  bytes and SHA-256 are unchanged; its licence is now recorded as Apache-2.0 with the
  Commons Clause.
• Update the model tables, installation notes, tests, and v0.2.0 changelog.

Validation

• ruff check src tests
• pytest -m "not slow and not network" -q: 738 passed, 10 skipped, 108 deselected
• Live revision and checksum-metadata checks for both GigaPath-Flash weights, plus the
  official HIPT Git LFS pointer: 4 passed
• Built the wheel and verified the new slide-encoder entry point is packaged
• GPU-validated on the DGX Spark's NVIDIA GB10 with PyTorch 2.12.0+cu130 and
  flash-attn 2.8.3.post1:
  • GigaPath-Flash tile: finite 384-dimensional outputs from the pinned weights
  • GigaPath-Flash slide: finite 384-dimensional output from the pinned LongNet weights
  • GigaPath-Flash and original GigaPath slide encoders: equivalent 20×-frame
    (runtime divisor 256) and 40×-frame (runtime divisor 512) coordinates produced
    identical positional indices and byte-identical real-weight outputs
  • Matched path: 16 transformed tiles produced (16, 384) features and a finite
    (384,) slide vector
  • HIPT: official checkpoint downloaded, SHA-verified before loading, and produced
    finite 384-dimensional outputs
  • Original GigaPath: revalidated with finite 1536-dimensional tile and
    768-dimensional slide outputs

This contributes to v0.2.0. It does not tag or publish the release.