Backport release/v6.7: Make the composite store router an atomic pointer

[seidroid]

Backport of #4470 to release/v6.7.

[cursor]

PR Summary

Medium Risk
Changes concurrency around the migration router on the state-commit hot path during write-mode transitions; logic is mostly pointer swapping and atomic loads, but incorrect ordering could still affect reads mid-migration.

Overview
CompositeCommitStore now holds its migration router in an atomic.Pointer instead of a plain field, with loadRouter / storeRouter as the only way to read or install it. All read/write paths (Get, ApplyChangeSets, SetWriteMode, buildRouter, close/cleanup, etc.) go through those helpers so SetWriteMode can swap the router while cached GetChildStoreByName views still resolve the current router on each operation (via cs.loadRouter passed into NewRouterCommitKVStore).

Adds TestComposite_Auto_ChildStoreReadsDuringWriteModeSwitch (intended for -race) to assert a child-store Get keeps returning the right value while a concurrent SetWriteMode(MigrateEVM) runs. Existing tests are updated to assert router presence through loadRouter() instead of touching the field directly.

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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	Oct 6, 2026, 2:04 PM

[seidroid]

This is a straight backport of #4470. It changes the composite store's router to an atomic.Pointer that is read and written only through loadRouter and storeRouter, so cached child-store views can read safely while SetWriteMode swaps the router. It adds a regression test that runs reads and a write-mode switch at the same time (meant for -race). Every former cs.router access is converted, sync/atomic is imported, the store is never copied by value, and the existing tests only change to call the new accessor.

Findings: 0 blocking | 0 non-blocking | 0 posted inline

Blockers

• None at the file/PR level.

Non-blocking

• None at the file/PR level.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.72%. Comparing base (7de88f1) to head (67b7662).

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/composite/store.go	95.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v6.7    #4473      +/-   ##
================================================
- Coverage         61.71%   60.72%   -0.99%     
================================================
  Files              2171     2075      -96     
  Lines            189923   178821   -11102     
================================================
- Hits             117206   108590    -8616     
+ Misses            61837    60243    -1594     
+ Partials          10880     9988     -892     

Flag	Coverage Δ
sei-db	69.80% <ø> (ø)
sei-db-state-db	?
sei-db-state-db-pr	70.43% <95.00%> (?)

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/composite/store.go	71.12% <95.00%> (+0.49%)	⬆️

... and 96 files with indirect coverage changes

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