Use composite SC for RootMultistore

[yzang2019]

Describe your changes and provide context

The purpose of this PR is to make sure sei-chain will use the new CompositeSC which manages both MemIAVL and FlatKV backend:

• Root multistore (CMS) now creates composite sc instead of memiavl sc
• Fixed a few functions for composite sc to make cosmos only mode behaves the same as before
• Adjust flatkv LoadVersion interface to remove readonly

Testing performed to validate your change

[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	Feb 19, 2026, 3:58 PM

[codecov]

Codecov Report

❌ Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.32%. Comparing base (8e0d3be) to head (9175e8e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/composite/store.go	68.75%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2930      +/-   ##
==========================================
- Coverage   57.34%   57.32%   -0.03%     
==========================================
  Files        2095     2095              
  Lines      172872   172870       -2     
==========================================
- Hits        99138    99101      -37     
- Misses      64848    64879      +31     
- Partials     8886     8890       +4     

Flag	Coverage Δ
sei-chain	52.82% <73.68%> (-0.03%)	⬇️
sei-cosmos	48.18% <100.00%> (-0.01%)	⬇️
sei-db	68.42% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-cosmos/storev2/rootmulti/store.go	41.96% <100.00%> (ø)
sei-db/state_db/sc/flatkv/store.go	65.17% <100.00%> (+1.14%)	⬆️
sei-db/state_db/sc/composite/store.go	52.25% <68.75%> (+3.60%)	⬆️

... and 31 files with indirect coverage changes

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[blindchaser]

it looks before we always create new store and now we mutates self if it's non readOnly.

can you explain a bit why