Keep semantic timestamp casts in physical comparison pruning

[osipovartem]

Summary

• Prevent physical cast unwrapping from dropping timestamp unit conversions or timezone attachment in comparisons and pruning.
• Share the semantic guard with the logical optimizer while preserving safe equal-unit timezone removal/replacement fast paths.
• Add a two-row-group pruning regression that previously dropped a matching row group.

Validation

• Focused tests: expr-common 166, optimizer 764, physical-expr 1609 (2 ignored), pruning 90.
• Clippy and rustfmt passed for touched crates.
• Independent read-only review approved correctness, performance, API, and test coverage. No GitHub self-review comments posted.