Add fallible AD rule hooks

[shinaoka]

Summary

• Add ADRuleError, ADRuleKind, and ADRuleResult for non-panicking AD rule failures.
• Add PrimitiveOp try_linearize and try_transpose_rule hooks while keeping existing infallible compatibility methods.
• Document and test unsupported-rule reporting.

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --release
• cargo test --doc --release

Implemented with OpenAI Codex.