fix(dash-spv): degrade gracefully when advisory file locking is unsupported (Android)

[QuantumExplorer]

Summary

Fixes #840.

On Android (target_os = "android") the Rust standard library does not implement flock-based advisory file locking — File::try_lock() is a compile-time stub that unconditionally returns ErrorKind::Unsupported. LockFile::new() treated any TryLockError::Error as fatal, so dash-spv storage init always failed with:

SPV error: Write failed: Failed to acquire lock: try_lock() not supported

This blocked the entire Core/SPV layer on Android — no header/filter sync, no balances, no L1 send. iOS, macOS, and Linux desktop were unaffected (their targets implement flock).

Fix

The lock file is only a best-effort guard against multiple processes sharing one data directory. On single-process platforms that lack advisory locking, proceeding without it is safe. This PR treats ErrorKind::Unsupported as non-fatal: skip the advisory lock with a tracing::warn! and continue.

• WouldBlock (directory already in use) → still fatal (DirectoryLocked).
• Genuine I/O errors → still fatal (WriteFailed).
• Unsupported → warn and continue without the cross-process guard.
• The fast path on desktop/iOS is unchanged.

Handling ErrorKind::Unsupported generically (rather than #[cfg(target_os = "android")]-gating) is more robust: it covers any non-flock target without special-casing.

Testing

The classification logic is extracted into a small classify_lock_result helper so the degradation path can be unit-tested with a synthesized Unsupported error — this never occurs naturally on Linux/macOS CI. Added tests cover all four outcomes:

• lock acquired → Ok(true)
• WouldBlock → DirectoryLocked
• Unsupported → Ok(false) (non-fatal, skip)
• genuine I/O error → WriteFailed

All existing lockfile tests continue to pass:

running 9 tests ... test result: ok. 9 passed; 0 failed

cargo fmt --check and cargo clippy -p dash-spv --all-targets --all-features are clean.

🤖 Generated with Claude Code

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Review details

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Run ID: 8e39222c-e230-410b-96a9-0c0ecb74849c

📥 Commits

Reviewing files that changed from the base of the PR and between b0a909c and e0410be.

📒 Files selected for processing (1)

• dash-spv/src/storage/lockfile.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/compassionate-sutherland-6f951e

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[codecov]

Codecov Report

❌ Patch coverage is 93.47826% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.38%. Comparing base (b0a909c) to head (e0410be).

Files with missing lines	Patch %	Lines
dash-spv/src/storage/lockfile.rs	93.47%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #841      +/-   ##
==========================================
- Coverage   73.44%   73.38%   -0.07%     
==========================================
  Files         324      324              
  Lines       72577    72616      +39     
==========================================
- Hits        53306    53291      -15     
- Misses      19271    19325      +54     

Flag	Coverage Δ
core	76.94% <ø> (ø)
ffi	46.84% <ø> (-1.14%)	⬇️
rpc	20.00% <ø> (ø)
spv	90.61% <93.47%> (+0.33%)	⬆️
wallet	72.61% <ø> (ø)

Files with missing lines	Coverage Δ
dash-spv/src/storage/lockfile.rs	94.78% <93.47%> (+1.36%)	⬆️

... and 23 files with indirect coverage changes