fix(flatkv): sync SNAPSHOT_BASE on WriteSnapshot to avoid unnecessary WAL catchup on restart

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 4, 2026, 4:46 PM

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[chatgpt-codex-connector]

Use the correct FlatKV root in snapshot-base test

TestWriteSnapshotUpdatesSnapshotBase initializes the store with NewCommitStore(dir, ...), but then reads SNAPSHOT_BASE from filepath.Join(dir, "data", flatkvRootDir, ...); NewCommitStore uses the provided dbDir directly (see store.go), so this path points to the wrong location and the read will fail with ENOENT, meaning the new regression test does not validate the behavior it intends to cover.

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Codecov Report

❌ Patch coverage is 20.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.28%. Comparing base (dcf5a0e) to head (6ceb6c9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/flatkv/snapshot.go	20.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3011      +/-   ##
==========================================
- Coverage   58.39%   58.28%   -0.12%     
==========================================
  Files        2109     2108       -1     
  Lines      174936   173733    -1203     
==========================================
- Hits       102161   101266     -895     
+ Misses      63758    63438     -320     
- Partials     9017     9029      +12     

Flag	Coverage Δ
sei-chain-pr	61.45% <20.00%> (?)
sei-db	70.41% <ø> (ø)

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Files with missing lines	Coverage Δ
sei-db/state_db/sc/flatkv/snapshot.go	66.16% <20.00%> (-0.71%)	⬇️

... and 34 files with indirect coverage changes

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