FlatKV Value Type Structs

[cody-littley]

Describe your changes and provide context

For each DB in FlatKV (other than metadata), we need to add 9 extra bytes of metadata. This PR introduces structs for each DB that facilitate this metadata addtion.

Testing performed to validate your change

Unit tests, ran full tests in branch where this code is integrated.

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	❌ failed (0)	Apr 3, 2026, 1:10 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.63%. Comparing base (3d64c1b) to head (c42bbb6).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3164      +/-   ##
==========================================
+ Coverage   58.55%   58.63%   +0.07%     
==========================================
  Files        2099     2105       +6     
  Lines      173847   174179     +332     
==========================================
+ Hits       101803   102135     +332     
  Misses      62873    62873              
  Partials     9171     9171              

Flag	Coverage Δ
sei-chain-pr	100.00% <100.00%> (?)
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/vtype/account_data.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/flatkv/vtype/base_types.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/flatkv/vtype/code_data.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/flatkv/vtype/legacy_data.go	100.00% <100.00%> (ø)
.../state_db/sc/flatkv/vtype/pending_account_write.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/flatkv/vtype/storage_data.go	100.00% <100.00%> (ø)

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[yzang2019]

IMO, this VersionStartPos, MetadataStartPos, MetadataLength can be a global const shared by all DB instead of redefine in each DB

[yzang2019]

Length can be hardcoded, but Position should be derived from some math calculation

[yzang2019]

I think for legacyDB, we probably don't want this extra 8 bytes to start with, because we could in the future store all cosmos module state to this legacyDB