Add fallible AD transform APIs

[shinaoka]

Summary

• Add try_differentiate, try_transpose, try_eager_transpose_fragment, and try_backward_dag APIs that propagate ADRuleError.
• Keep existing infallible APIs as compatibility wrappers.
• Thread fallible eager transpose through BackwardCallbacks and add regression coverage.

Dependency

• Depends on Add fallible AD rule hooks chainrules-rs#13, now merged. Cargo pins chainrules rev bc46e8d3745f85cc8047fd8e7b484f8e5ed54d3c.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo nextest run --release --workspace --no-fail-fast
• cargo test --doc --release --workspace
• cargo llvm-cov nextest --workspace --release --json --output-path coverage.json
• python3 scripts/check-coverage.py coverage.json
• cargo doc --workspace --no-deps
• python3 scripts/check-docs-site.py

Implemented with OpenAI Codex.