Snapshot write rate testing

[stevenlanders]

Describe your changes and provide context

This PR introduces an optional I/O rate limiter for snapshot writes, configurable via sc-snapshot-write-rate-mbps. The primary goal is to prevent page cache eviction and improve stability on machines with limited RAM by capping the total write bandwidth during snapshot creation.

It implements a global token bucket rate limiter shared across all trees and files, ensuring the total I/O is truly capped regardless of parallelism. The default value of 0 means unlimited, preserving existing behavior and ensuring backward compatibility.

Testing performed to validate your change

• Added a new unit test, TestGlobalRateLimiterSharedAcrossWriters, to validate shared limiter behavior and timing.
• Updated TestWriteSnapshotWithBuffer to pass a nil limiter (unlimited).
• All sei-db tests, integration tests, and linting passed.
• Noted and confirmed a pre-existing, unrelated CI compilation error in storev2/rootmulti/store_test.go which is not caused by this PR.
• Real-world testing is ongoing on an RPC node and planned for sei-0/sei-1 validators.

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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 5, 2026, 1:32 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.62%. Comparing base (3d6356d) to head (9791e38).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2803      +/-   ##
==========================================
+ Coverage   46.97%   56.62%   +9.65%     
==========================================
  Files        1965     2031      +66     
  Lines      160525   165854    +5329     
==========================================
+ Hits        75410    93922   +18512     
+ Misses      78585    63686   -14899     
- Partials     6530     8246    +1716     

Flag	Coverage Δ
sei-chain	41.50% <ø> (-0.05%)	⬇️
sei-cosmos	48.13% <ø> (+<0.01%)	⬆️
sei-db	68.72% <ø> (ø)
sei-tendermint	57.96% <ø> (?)

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see 320 files with indirect coverage changes

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