feat: add configurable io rate limiting for snapshot writes

[blindchaser]

Describe your changes and provide context

Background

v6.3 optimized snapshot creation time from >3 hours to ~20 minutes using aggressive parallel writes. While this improves performance on high-end machines, it can cause high I/O bursts that may impact page cache efficiency on systems with limited RAM.

Solution

Add optional I/O rate limiting for snapshot writes:

• New config: sc-snapshot-write-rate-mbps (default: 0 = unlimited)
• Global limiter shared across all trees/files using token bucket algorithm
• Allows operators to trade snapshot speed for more stable I/O patterns

Configuration Example

[state-commit]
sc-snapshot-write-rate-mbps = 300

[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 4, 2026, 7:49 PM

[codecov]

Codecov Report

❌ Patch coverage is 81.66667% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.95%. Comparing base (c68f987) to head (979adf7).

Files with missing lines	Patch %	Lines
sei-db/sc/memiavl/snapshot.go	84.61%	3 Missing and 3 partials ⚠️
sei-db/sc/memiavl/multitree.go	70.00%	1 Missing and 2 partials ⚠️
sei-db/config/config.go	0.00%	1 Missing ⚠️
sei-db/sc/store.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v6.3    #2802      +/-   ##
================================================
+ Coverage         45.17%   45.95%   +0.78%     
================================================
  Files               853     1199     +346     
  Lines             70076   104465   +34389     
================================================
+ Hits              31654    48011   +16357     
- Misses            35760    52224   +16464     
- Partials           2662     4230    +1568     

Flag	Coverage Δ
sei-chain	42.53% <100.00%> (+<0.01%)	⬆️
sei-db	45.15% <81.35%> (+0.29%)	⬆️
sei-ibc-go	55.96% <ø> (ø)
sei-tendermint	47.50% <ø> (?)
sei-wasmd	41.56% <ø> (ø)
sei-wasmvm	39.88% <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	83.01% <100.00%> (+0.32%)	⬆️
sei-db/sc/memiavl/db.go	63.33% <100.00%> (+0.12%)	⬆️
sei-db/sc/memiavl/opts.go	100.00% <ø> (ø)
sei-db/config/config.go	0.00% <0.00%> (ø)
sei-db/sc/store.go	0.00% <0.00%> (ø)
sei-db/sc/memiavl/multitree.go	79.09% <70.00%> (-0.32%)	⬇️
sei-db/sc/memiavl/snapshot.go	58.44% <84.61%> (+1.62%)	⬆️

... and 348 files with indirect coverage changes

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