Expose trace storage traits for custom sequential runners

[twiecki]

Custom sequential runners can create a chain through the public API, but cannot use the existing Arrow (or other) trace backends because StorageConfig, TraceStorage, and ChainStorage are hidden behind a private module. Extracting per-draw statistics also needs the unexported StatsDims.

Re-export those existing types without changing their implementations. This lets a WASM runner call expanded_draw, record the returned values and statistics through ArrowConfig, and produce the same Arrow trace used by Nutpie instead of implementing a separate storage backend.

Validated with an external-crate regression test (cargo test --no-default-features --features arrow --test public_storage_api) and a release WASM adapter that records and serializes the resulting traces. The browser integration lives separately in https://github.com/pymc-labs/nuts-rs-wasm.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.85%. Comparing base (394c629) to head (b6f058e).
⚠️ Report is 158 commits behind head on main.

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HEAD has 1 upload less than BASE

Flag	BASE (394c629)	HEAD (b6f058e)
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Additional details and impacted files

@@             Coverage Diff             @@
##             main      #77       +/-   ##
===========================================
- Coverage   83.77%   66.85%   -16.93%     
===========================================
  Files           8       34       +26     
  Lines        1923     9413     +7490     
===========================================
+ Hits         1611     6293     +4682     
- Misses        312     3120     +2808     

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