chore(spanner): add DATE and TIMESTAMP codecs for key recipe routing

[olavloite]

Add a standalone temporal codecs module (routing::temporal) to support Spanner DATE and TIMESTAMP key column evaluation in location-aware routing. This implementation parses ISO 8601 date strings into days since Unix epoch and RFC 3339 timestamp strings into UTC offset seconds and nanoseconds, converting them into Spanner's ordered binary format (ssformat). The module includes full support for valid Spanner date ranges (0001-01-01 to 9999-12-31), non-UTC timezone offsets, leap second normalization, and verification of chronological sort order preservation.

[gemini-code-assist]

Code Review

This pull request introduces a new temporal module in the Spanner routing crate, implementing codecs for DATE and TIMESTAMP data types to support location-aware routing. It includes robust parsing and encoding functions to convert ISO 8601 dates and RFC 3339 timestamps into Spanner's binary representation, along with comprehensive unit tests covering edge cases like leap years, timezone offsets, and chronological sorting. No review comments were provided, and the implementation looks solid and well-tested, so I have no additional feedback.

[codecov]

Codecov Report

❌ Patch coverage is 99.31034% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.25%. Comparing base (0df927e) to head (e5fb5e4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/routing/temporal.rs	99.31%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6288      +/-   ##
==========================================
+ Coverage   96.23%   96.25%   +0.01%     
==========================================
  Files         280      281       +1     
  Lines       72146    72436     +290     
==========================================
+ Hits        69433    69724     +291     
+ Misses       2713     2712       -1     

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