One call to ApplyChangeSets per block

[cody-littley]

Describe your changes and provide context

Things can be simplified greatly (in future PRs) if we limit calls to ApplyChangeSets() to once per block. Current code calls this method with empty change sets sometimes, and the code required to support this is extremely nontrivial.

[github-actions]

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Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jun 2, 2026, 4:56 PM

[codecov]

Codecov Report

❌ Patch coverage is 91.54930% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.68%. Comparing base (f8f1af1) to head (8dfea0c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/migration/router_builder.go	89.79%	10 Missing ⚠️
sei-cosmos/storev2/rootmulti/store.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3533      +/-   ##
==========================================
- Coverage   59.12%   58.68%   -0.45%     
==========================================
  Files        2213     2170      -43     
  Lines      182774   178142    -4632     
==========================================
- Hits       108072   104537    -3535     
+ Misses      64985    64223     -762     
+ Partials     9717     9382     -335     

Flag	Coverage Δ
sei-chain-pr	62.88% <77.77%> (?)
sei-db	70.41% <ø> (ø)
sei-db-state-db	?
sei-db-state-db-pr	77.25% <92.48%> (?)

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.go	76.94% <ø> (ø)
sei-db/state_db/sc/flatkv/store_apply.go	91.78% <100.00%> (+0.12%)	⬆️
sei-db/state_db/sc/flatkv/store_write.go	76.92% <100.00%> (-1.13%)	⬇️
...ei-db/state_db/sc/migration/accumulating_writer.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/migration/flushing_router.go	100.00% <100.00%> (ø)
sei-cosmos/storev2/rootmulti/store.go	65.21% <77.77%> (+0.12%)	⬆️
sei-db/state_db/sc/migration/router_builder.go	62.92% <89.79%> (+6.20%)	⬆️

... and 46 files with indirect coverage changes

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