Add FilesystemStoreV2Error for v1 data detection

[benthecarman]

FilesystemStoreV2::new previously returned io::Error with ErrorKind::InvalidData when the data directory contained top-level files left behind by FilesystemStore (v1). That forced us to match on an error that could potentially be given by our normal io calls. This adds a dedicated FilesystemStoreV2Error enum with a V1DataDetected(PathBuf) so we can distinguish between normal io errors and an old V1 fs store.

suggested from: lightningdevkit/ldk-node#872 (comment)

[ldk-reviews-bot]

👋 Thanks for assigning @tnull as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've reviewed the entire diff carefully. The change is well-scoped: it introduces a dedicated FilesystemStoreV2Error enum to replace the generic io::Error with ErrorKind::InvalidData, the Display/Error/From implementations are correct, the constructor properly uses ? for io::Error propagation via the From impl, and the test is updated to match on the specific enum variant rather than the error kind.

No issues found.

[codecov]

Codecov Report

❌ Patch coverage is 20.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.02%. Comparing base (5079a0b) to head (6112521).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-persister/src/fs_store/v2.rs	20.00%	16 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4573   +/-   ##
=======================================
  Coverage   87.01%   87.02%           
=======================================
  Files         161      161           
  Lines      108972   108979    +7     
  Branches   108972   108979    +7     
=======================================
+ Hits        94827    94836    +9     
+ Misses      11660    11657    -3     
- Partials     2485     2486    +1     

Flag	Coverage Δ
fuzzing-fake-hashes	30.82% <0.00%> (-0.01%)	⬇️
fuzzing-real-hashes	22.66% <0.00%> (+<0.01%)	⬆️
tests	86.13% <20.00%> (+<0.01%)	⬆️

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[tnull]

LGTM

[ldk-reviews-bot]

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