chore: copy corrupted WAL and docs to .broken dir

[cheb0]

Description

If WAL file is corrupted, then increment a metric and copy to a separate directory .broken

---

• I have read and followed all requirements in CONTRIBUTING.md;
• I used LLM/AI assistance to make this pull request;

[codecov-commenter]

Codecov Report

❌ Patch coverage is 19.04762% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.60%. Comparing base (0e194d8) to head (6ee8a1a).

Files with missing lines	Patch %	Lines
frac/active.go	8.69%	20 Missing and 1 partial ⚠️
util/fs.go	0.00%	11 Missing ⚠️
storage/wal_reader.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #395      +/-   ##
==========================================
- Coverage   70.73%   70.60%   -0.14%     
==========================================
  Files         219      219              
  Lines       16930    16967      +37     
==========================================
+ Hits        11976    11979       +3     
- Misses       4059     4091      +32     
- Partials      895      897       +2     

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