Add extension AD rule registry

[shinaoka]

Summary

• Add a separate ExtensionAdRule registry for out-of-tree tensor operations, with fallible linearize/transpose dispatch through chainrules/tidu.
• Re-export registration helpers and ExtensionFamilyId, and add a proc-macro crate for stable family IDs.
• Add traced and eager error propagation for missing extension AD rules, plus facade tests for registered-rule AD and validation paths.
• Update the extension-op spec to describe registered AD rules and same-graph extension composition.

Dependencies

• Depends on Add fallible AD rule hooks chainrules-rs#13, merged as da3f2b50ba2e6a0738737e4e4b2ec8f5c8243469.
• Depends on Add fallible AD transform APIs tidu-rs#22 and fix: update dep graph generator for directory layout #23, latest merged rev 1d4a4c1d1c3923b79a509467ea4af4be2a148aaf.

Validation

• cargo fmt --all --check
• cargo nextest run --workspace --release --no-fail-fast
• cargo test --doc --workspace --release
• 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
• bash scripts/check-repo-settings.sh

Implemented with OpenAI Codex.