fix: subdirectory dated files decay by filename date

[sachinsharma9780]

Summary:

• What was broken (all memory// files were evergreen regardless of name)
• Root cause (_DATED_PATH_RE only matched root-level dated files)
• Fix (regex extended with (?:[^/]+/)? to match one optional subdirectory level)
• Design rationale (filename convention = self-documenting, no mtime needed)
• Tests added (unit + integration)

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.18%. Comparing base (5abcb96) to head (66a0116).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main       #2   +/-   ##
=======================================
  Coverage   88.18%   88.18%           
=======================================
  Files          25       25           
  Lines        1676     1676           
=======================================
  Hits         1478     1478           
  Misses        198      198           

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