Add per-asset dataStandard, HED standard, and extensions to StandardsType

[yarikoptic]

Summary

• Add dataStandard: Optional[List[StandardsType]] field to BareAsset so dandi-cli can declare per-asset data standards (NWB, BIDS, HED, OME/NGFF) that flow through to AssetsSummary during aggregation
• Add version: Optional[str] field to StandardsType to track standard versions (e.g. NWB 2.7.0, BIDS 1.9.0, HED 8.2.0)
• Add extensions: Optional[List[StandardsType]] self-referencing field to StandardsType for standard extensions (NWB ndx-* extensions, HED library schemas)
• Add hed_standard constant alongside existing nwb_standard, bids_standard, ome_ngff_standard
• Collect per-asset dataStandard entries in aggregate_assets_summary(), with deprecated heuristic fallbacks for older clients (remove after 2026-12-01)
• Register dataStandard in migrate() _FIELDS_INTRODUCED for downgrade pruning
• Comprehensive CLAUDE.md with architecture docs and schema change checklist

Companion dandi-cli PR: dandi/dandi-cli#XXXX

Test plan

• test_hed_standard_structure — verifies hed_standard has correct name, identifier, schemaKey
• test_aggregate_per_asset_datastandard — HED declared per-asset flows to summary
• test_aggregate_per_asset_datastandard_no_duplication — BIDS not duplicated when declared both per-asset and via heuristic
• All 243 existing tests pass

PR in dandi-cli:

• Populate per-asset dataStandard for BIDS, HED, NWB, and extensions dandi-cli#1811

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.82%. Comparing base (07aa22c) to head (4bb5f6f).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
dandischema/metadata.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #371      +/-   ##
==========================================
- Coverage   97.91%   97.82%   -0.10%     
==========================================
  Files          18       18              
  Lines        2401     2432      +31     
==========================================
+ Hits         2351     2379      +28     
- Misses         50       53       +3     

Flag	Coverage Δ
unittests	97.82% <90.90%> (-0.10%)	⬇️

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