add passthrough router

[cody-littley]

Describe your changes and provide context

When integrating routers with composite.Store, I realized there was an edge case we hadn't considered. There are test apps (e.g. SimApp) that use different module names than expected. In order to keep those test apps working, I needed to introduce the PassthroughRouter, which skips all module name checking. We can use the passthrough router when operating with a single database backend, since we really only care about restricting module names during migration.

Testing performed to validate your change

unit tests

---

Note

Medium Risk
Changes routing behavior for MemiavlOnly/FlatKVOnly to stop validating module names, which could alter error surfaces and allow unexpected store names to reach the backend. Scope is localized to the migration router layer and covered by new unit tests.

Overview
Adds a new PassthroughRouter that forwards Read, ApplyChangeSets, Iterator, and GetProof directly to provided accessors (and returns explicit errors when iteration/proofs are unsupported).

Updates BuildRouter to use this passthrough behavior in MemiavlOnly and FlatKVOnly modes (instead of ModuleRouter name checks) to keep compatibility with apps that use nonstandard store names, and generalizes flatkv wiring by accepting flatkv.Store rather than *flatkv.CommitStore. Includes comprehensive unit tests asserting name-agnostic forwarding and error propagation.

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[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, 1:19 PM

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.36%. Comparing base (2203f0c) to head (a77ea81).

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/migration/router_builder.go	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3430      +/-   ##
==========================================
+ Coverage   59.28%   59.36%   +0.08%     
==========================================
  Files        2118     2113       -5     
  Lines      175556   174387    -1169     
==========================================
- Hits       104078   103528     -550     
+ Misses      62407    61851     -556     
+ Partials     9071     9008      -63     

Flag	Coverage Δ
sei-chain-pr	80.53% <95.00%> (?)
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/migration/migration_manager.go	95.48% <ø> (ø)
sei-db/state_db/sc/migration/passthrough_router.go	100.00% <100.00%> (ø)
sei-db/state_db/sc/migration/router_builder.go	52.31% <88.23%> (+1.98%)	⬆️

... and 50 files with indirect coverage changes

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