[codex] remove legacy crates, add HVP support, and fix oracle replay

[shinaoka]

What changed

• removed the legacy extension and internal crates that no longer match the current workspace shape, and aligned top-level docs and coverage config with the remaining public surface
• added higher-order AD plumbing for traced tensors, including extra-root propagation and linalg rule fixes needed for HVP support
• added an oracle replay test harness for supported ops and fixed the tenferro-side linalg and zero-size AD/runtime bugs it exposed

Why this changed

The branch consolidates the workspace around the current tensor/frontend stack, then hardens AD correctness against oracle-based regression checks. The replay work is intentionally fixing tenferro itself rather than adding downstream skips for supported operations.

User and developer impact

• higher-order AD paths such as HVP are now covered by regression tests
• linalg APIs handle batched vector right-hand sides and zero-sized cases more consistently
• supported oracle replay cases now run end-to-end, which gives a stronger regression gate for future AD and linalg changes

Root causes fixed

• traced higher-order derivatives were dropping graph roots needed for nested differentiation
• several linalg JVP/VJP paths had semantic mismatches, especially around triangular solve, QR, and structured matrix handling
• zero-sized matmul, masking, and batched linalg paths could panic or segfault instead of returning valid empty outputs

Validation

• cargo fmt --all --check
• cargo test --workspace --release
• cargo llvm-cov --workspace --json --output-path coverage.json
• python3 scripts/check-coverage.py coverage.json
• cargo doc --workspace --no-deps
• python3 scripts/check-docs-site.py