Use datalad-fuse's remfile backend when available

[CodyCBakerPhD]

Stacked on #1956. This tries the development version of datalad-fuse from datalad/datalad-fuse#131, which adds pluggable backends, including remfile for HDF5-based files.

Changes

• Adapter choice: get_adapter() uses datalad_fuse.adapter.RemoteFilesystemAdapter (dandiset.yaml: should become READONLY locally, with a comment, no metadata uploaded from it #131) when it is available, and falls back to the released datalad_fuse.fsspec.FsspecAdapter (0.5.1) otherwise.
  • With dandiset.yaml: should become READONLY locally, with a comment, no metadata uploaded from it #131 and remfile installed, NWB and other HDF5 files are read through remfile; other files still go through fsspec.
  • The backends can be configured with DataLad's datalad.fusefs.backends setting.
• S3 export fallback: get_annexed_readable() also tries the S3 exporttree URLs from dandiset.yaml: should become READONLY locally, with a comment, no metadata uploaded from it #131 when git-annex lists no URL, for older datasets that lack versioned S3 URLs.
• Docs: a note in the validate docs.
• Tests: test_annexed_readable_remfile checks that an NWB file is opened through datalad-fuse's RemfileWrapper and read with h5py over the local HTTP range server. It is skipped unless both dandiset.yaml: should become READONLY locally, with a comment, no metadata uploaded from it #131 and remfile are installed.
• CI: a new datalad-dev job installs git-annex, remfile and datalad-fuse from refs/pull/131/head. The existing datalad job keeps testing the PyPI release.

Locally, the datalad-fuse and validate stream tests pass with both datalad-fuse 0.5.1 (where the remfile test is skipped) and #131 plus remfile.

Once #131 is released, the CI install can switch to that release and the datalad extra's lower bound can be raised.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Qn5WBSiQgoZoL4fytF6nEr

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Generated by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 10.81081% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.10%. Comparing base (a65f6ee) to head (bb56977).

Files with missing lines	Patch %	Lines
dandi/support/tests/test_datalad_fuse.py	9.52%	19 Missing ⚠️
dandi/support/datalad_fuse.py	12.50%	14 Missing ⚠️

Additional details and impacted files

@@                             Coverage Diff                             @@
##           claude/validate-stream-via-datalad-fuse    #1957      +/-   ##
===========================================================================
- Coverage                                    91.13%   88.10%   -3.04%     
===========================================================================
  Files                                           94       94              
  Lines                                        14806    14837      +31     
===========================================================================
- Hits                                         13494    13072     -422     
- Misses                                        1312     1765     +453     

Flag	Coverage Δ
unittests	88.10% <10.81%> (-3.04%)	⬇️

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