composite.Store uses routers

[cody-littley]

Describe your changes and provide context

Integrates migration.Router with composite.Store implementation.

Testing performed to validate your change

Unit tests. More unit tests to follow in the next PR.

[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	May 14, 2026, 4:54 PM

[codecov]

Codecov Report

❌ Patch coverage is 73.44498% with 111 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.42%. Comparing base (0f16b6d) to head (1758c2b).

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/composite/store.go	70.65%	46 Missing and 35 partials ⚠️
sei-db/state_db/sc/flatkv/store_meta.go	63.93%	13 Missing and 9 partials ⚠️
sei-cosmos/storev2/rootmulti/store.go	20.00%	2 Missing and 2 partials ⚠️
sei-db/config/sc_config.go	50.00%	1 Missing and 1 partial ⚠️
sei-db/state_db/sc/migration/router_builder.go	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3420      +/-   ##
==========================================
+ Coverage   59.23%   59.42%   +0.18%     
==========================================
  Files        2116     2115       -1     
  Lines      175440   174119    -1321     
==========================================
- Hits       103916   103462     -454     
+ Misses      62460    61641     -819     
+ Partials     9064     9016      -48     

Flag	Coverage Δ
sei-chain-pr	68.90% <73.44%> (?)
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
app/seidb.go	76.81% <ø> (+4.81%)	⬆️
sei-cosmos/server/config/config.go	94.35% <100.00%> (-0.07%)	⬇️
sei-db/config/write_mode.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/memiavl/store.go	92.59% <100.00%> (-0.06%)	⬇️
sei-db/state_db/sc/migration/migration_manager.go	95.48% <100.00%> (ø)
sei-db/state_db/sc/migration/passthrough_router.go	100.00% <100.00%> (ø)
sei-db/config/sc_config.go	87.50% <50.00%> (-12.50%)	⬇️
sei-db/state_db/sc/migration/router_builder.go	56.29% <92.00%> (+5.95%)	⬆️
sei-cosmos/storev2/rootmulti/store.go	64.30% <20.00%> (-0.50%)	⬇️
sei-db/state_db/sc/flatkv/store_meta.go	68.80% <63.93%> (-4.64%)	⬇️
... and 1 more

... and 119 files with indirect coverage changes

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