Enforce recorded model revision pins

[CraigMyles]

Summary

• pass each registry weights_revision to Hugging Face loaders whose API accepts a
  revision
• for CONCH, KRONOS, OpenCLIP, and MADELEINE, download the pinned revision first and
  give the upstream factory a local path
• verify the recorded SHA-256 before those local checkpoints reach an upstream
  deserializer
• pin BiomedCLIP's nested PubMedBERT configuration to an immutable revision and make
  both model construction and tokenizer configuration resolve locally
• pin every mutable git+https installation example to the audited upstream commit
• require OpenCLIP 3.x for its supported local-dir: loading contract

Why

The registry already records immutable model revisions, but several loaders and install
examples did not enforce them. A store could therefore record one revision while an
upstream factory or a copied install command resolved the repository's current default
branch.

BiomedCLIP also contains a transitive dependency: its pinned top-level
open_clip_config.json names a separate mutable PubMedBERT repository. OpenCLIP builds
that text tower even though raw2features exposes only encode_image, so the nested
configuration must be pinned as well.

Implementation

• ClipHFEmbedder forwards revision= to AutoModel.from_pretrained.
• CONCH and KRONOS download their exact checkpoint files at the registry revision,
  verify their registry digest, and pass only the verified local path upstream.
• QuiltNet and BiomedCLIP use a raw2features-owned, model-and-revision-specific cache
  containing only open_clip_config.json and open_clip_pytorch_model.bin; the exact
  .bin is verified before OpenCLIP loads local-dir:<path>.
• BiomedCLIP downloads config.json, tokenizer_config.json, and vocab.txt from
  microsoft/BiomedNLP-BiomedBERT-base-uncased-abstract at
  d673b8835373c6fa116d6d8006b33d48734e305d, then atomically rewrites the app-owned
  OpenCLIP config so both nested names are absolute local paths. The shared Hugging Face
  cache is never modified.
• MADELEINE preserves RAW2FEATURES_MADELEINE_DIR and the existing default cache
  behavior, but downloads model_config.json and model.pt at the recorded revision,
  verifies model.pt, and calls the upstream local-path factory on the requested
  device.
• VCS install instructions now pin CONCH, KRONOS, MUSK, MADELEINE, and Prov-GigaPath;
  the already-pinned SEAL example now also pins its CONCH backbone example.

Compatibility

There is no CLI or embeddings-store schema change. Model outputs, preprocessing, and
recorded provenance fields are unchanged; the existing pins are now enforced during
download and installation. OpenCLIP users need open_clip_torch>=3.0, which provides
the local-directory loader used here.

Validation

• 333 passed, 50 deselected in the full non-slow suite after rebasing onto merged
  PR Fix multi-grid standalone slide embedding #7, on Linux/Python 3.12 with the optional dependencies installed
• 22 passed in the focused hub and loader-pinning tests
• ruff check .
• git diff --check
• independent review specifically checked the upstream loader contracts, pinned VCS
  revisions, transitive BiomedCLIP dependency, local-cache isolation, and
  deserialize-before-verification boundary; its blocking findings were fixed before
  the final test run
• no gated real-weight/GPU run was performed for this branch; loader contracts are
  covered with isolated tests and upstream-source review

Closes #5
Part of #6