fix(seidb-tool): addresses two issues against the FlatKV seidb tooling

[blindchaser]

Summary

This PR addresses two auditor findings against the FlatKV seidb tooling: snapshot clones could silently degrade to multi-GB byte-copies on tmpfs, and a snapshot/WAL race let catchup advance over missing versions without complaint. The fixes pin the tool clone to the source filesystem, validate WAL coverage after the clone, and make catchup enforce strict continuity from committedVersion + 1.

• sei-db/tools/cmd/seidb/operations/flatkv_open.go: places the temp clone as a sibling of dbDir so os.Link keeps working when $TMPDIR is tmpfs; replaces the EXDEV byte-copy fallback for snapshot files with a fatal error so a misconfigured deployment cannot silently RAM-copy multi-GB snapshots; introduces errSourceChurning and verifyClonedWALCovers to detect the race where a live writer rolls a new snapshot and front-truncates the WAL between our snapshot and changelog clone steps; routes that race through the existing retry loop alongside os.ErrNotExist. Changelog files keep their byte-copy semantics (WAL recovery may rewrite a corrupted tail).
• sei-db/state_db/sc/flatkv/store_catchup.go: makes catchup reject WAL gaps loudly. When committedVersion > 0, walFirstVer must be <= committedVersion + 1 or catchup returns an error instead of silently jumping forward and corrupting committedLtHash. Adds an inner expectedNext check inside the replay callback so an internal hole in the WAL (corruption or external surgery) is also rejected. Returns clean no-op when the WAL is entirely behind committedVersion.

Test plan

• sei-db/tools/cmd/seidb/operations/flatkv_open_test.go: covers same-filesystem placement of the tooling clone (TestPrepareFlatKVToolingClonePlacesTempDirOnSameFilesystem) and the snapshot/WAL truncation race (TestPrepareFlatKVToolingCloneDetectsWALTruncationRace), asserting that the latter surfaces errSourceChurning so the retry loop can re-select the snapshot. Existing retry-on-ENOENT, current-symlink-missing, and historical-height tests continue to pass.
• sei-db/state_db/sc/flatkv/store_catchup_test.go: TestCatchupRejectsWALGap builds a v1..v5 store, front-truncates the WAL to start at v4, rewinds committedVersion to v2, and asserts catchup returns a WAL gap error instead of silently advancing. TestCatchupNoOpWhenWALBehindCommittedVersion guards the steady-state case where the WAL is entirely behind committedVersion.
• Verified with gofmt -s -l (clean) and go test ./sei-db/state_db/... ./sei-db/tools/cmd/seidb/... ./sei-cosmos/storev2/rootmulti/... -run "Flatkv|FlatKV" (all green).

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	May 4, 2026, 5:51 PM

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[codecov]

Codecov Report

❌ Patch coverage is 61.76471% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.13%. Comparing base (4ad975b) to head (6a4388b).

Files with missing lines	Patch %	Lines
sei-db/tools/cmd/seidb/operations/flatkv_open.go	62.26%	12 Missing and 8 partials ⚠️
sei-db/state_db/sc/flatkv/store_catchup.go	60.00%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3337      +/-   ##
==========================================
+ Coverage   59.07%   59.13%   +0.05%     
==========================================
  Files        2099     2097       -2     
  Lines      172988   172326     -662     
==========================================
- Hits       102195   101903     -292     
+ Misses      61922    61556     -366     
+ Partials     8871     8867       -4     

Flag	Coverage Δ
sei-chain-pr	55.00% <61.76%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/state_db/sc/flatkv/store_catchup.go	60.36% <60.00%> (-0.06%)	⬇️
sei-db/tools/cmd/seidb/operations/flatkv_open.go	65.97% <62.26%> (-1.58%)	⬇️

... and 62 files with indirect coverage changes

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[cody-littley]

Not a blocker for this PR. As a future task, we should evaluate and update the flatKV snapshot threading model so that such races are not possible. Even if we're safe now, having to think about potential races makes safety a lot harder to reason about.