chore: improve findutils maintenance path

[vip892766gma]

Summary:

• Add edge-case unit tests for the size matcher (e.g., zero-byte files, exact boundary values across units c/w/b/k/M/G, and +/- prefix behavior) and the empty matcher (empty directory vs empty file, non-readable directory error path). Tests live in the same files via #[cfg(test)] modules, so no cross-module changes are needed.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[codecov]

Codecov Report

❌ Patch coverage is 92.78351% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.87%. Comparing base (c944cf7) to head (2a12b5b).

Files with missing lines	Patch %	Lines
src/find/matchers/empty.rs	85.41%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #687      +/-   ##
==========================================
+ Coverage   91.76%   91.87%   +0.11%     
==========================================
  Files          31       31              
  Lines        6206     6303      +97     
  Branches      328      329       +1     
==========================================
+ Hits         5695     5791      +96     
  Misses        390      390              
- Partials      121      122       +1     

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