[justinchuby]

Summary

• convert the remaining primary, draft, MTP, and EAGLE-3 model session resources to structurally shared Arc<Session> ownership
• add a named borrowed/shared ORT session owner for persistent decode runners, eliminating the synthetic 'static session borrow
• keep every IoBinding, allocator, device Value, KV/cache, and graph-capture state runner/worker-local
• preserve Session::supports_concurrent_run() as a capability signal and add stable-address, lifecycle, and W=1 continuation parity coverage
• update SESSION_CONCURRENCY.md only for this completed prerequisite

Scope

This PR is a shared immutable ORT ownership prerequisite only. It does not enable N>1 workers, concurrent session runs, new server worker configuration, or concurrent use of providers/configurations that decline supports_concurrent_run().

Validation

• cargo test -p onnx-genai-engine --lib
• cargo test -p onnx-genai-engine --lib --features native-backend
• cargo test -p onnx-genai-engine --test multi_session --test canonical_execution_parity --test chained_proposer_real
• cargo test -p onnx-genai-ort
• cargo test -p onnx-genai-server --lib
• cargo check -p onnx-genai-server --all-targets
• cargo clippy -p onnx-genai-ort -p onnx-genai-engine -p onnx-genai-server --all-targets -- -D warnings
• cargo clippy -p onnx-genai-engine -p onnx-genai-server --all-targets --features native-backend -- -D warnings
• cargo fmt --all -- --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.70%. Comparing base (74e6cb6) to head (af7b1ab).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2122      +/-   ##
==========================================
+ Coverage   80.29%   80.70%   +0.40%     
==========================================
  Files         426      429       +3     
  Lines      205244   215150    +9906     
  Branches   205244   215150    +9906     
==========================================
+ Hits       164801   173636    +8835     
- Misses      34792    35742     +950     
- Partials     5651     5772     +121     

Flag	Coverage Δ
cli-ort-linux	72.51% <ø> (?)
cli-ort-windows	72.10% <ø> (+0.09%)	⬆️
mlas	85.90% <ø> (?)
offline	80.82% <ø> (+0.31%)	⬆️

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see 64 files with indirect coverage changes

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