Fix time macro include digest

[fallintoplace]

Summary

• stop passing the cached result key into include validation
• seed the normalized time-macro include digest from the include-file digest
• add a regression test for __TIMESTAMP__ include digest updates

Why

The time-macro path reused a variable named digest from result_matches, but that value was the cached result key, not the include file digest. Timestamp metadata updates could therefore store a digest derived from the wrong input.

Testing

• cargo fmt --check
• cargo test -q compiler::preprocessor_cache::test::test_preprocessor_cache_timestamp_digest_uses_include_file_digest --lib
• cargo test -q compiler::preprocessor_cache --lib
• cargo test -q --lib

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.62%. Comparing base (bdbab18) to head (2f0cdf8).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/preprocessor_cache.rs	83.33%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2741      +/-   ##
==========================================
+ Coverage   74.56%   74.62%   +0.05%     
==========================================
  Files          70       70              
  Lines       39795    39833      +38     
==========================================
+ Hits        29675    29725      +50     
+ Misses      10120    10108      -12     

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