Add 9 bytes of metadata to FlatKV store

[cody-littley]

Describe your changes and provide context

In order to support BUD proofs, we need to add "block last modified" metadata to each value. Since we are about to do a migration, it's lower effort in the long run to add this metadata now instead of later.

Testing performed to validate your change

Unit tests

[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	✅ passed	Apr 1, 2026, 8:41 PM

[codecov]

Codecov Report

❌ Patch coverage is 85.54070% with 119 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.63%. Comparing base (232fee5) to head (2050921).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/flatkv/store_write.go	67.42%	26 Missing and 17 partials ⚠️
sei-db/state_db/sc/flatkv/store_read.go	75.80%	21 Missing and 9 partials ⚠️
sei-db/state_db/sc/flatkv/store_apply.go	83.53%	19 Missing and 8 partials ⚠️
sei-db/state_db/sc/flatkv/exporter.go	62.06%	9 Missing and 2 partials ⚠️
sei-db/state_db/sc/flatkv/iterator.go	50.00%	4 Missing and 1 partial ⚠️
sei-db/common/utils/hash64.go	85.71%	1 Missing and 1 partial ⚠️
sei-db/common/rand/canned_random.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3163      +/-   ##
==========================================
+ Coverage   58.57%   58.63%   +0.06%     
==========================================
  Files        2099     2108       +9     
  Lines      173853   174359     +506     
==========================================
+ Hits       101829   102244     +415     
- Misses      62864    62925      +61     
- Partials     9160     9190      +30     

Flag	Coverage Δ
sei-chain-pr	71.49% <85.54%> (?)
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/config.go	53.33% <100.00%> (+0.63%)	⬆️
sei-db/state_db/sc/flatkv/flatkv_test_config.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/flatkv/keys.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/flatkv/store.go	70.87% <100.00%> (ø)
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)
... and 7 more

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