Fix buffer offset in ProposerPriorityHash (CON-200)

[wen-coding]

Summary

• Fix binary.PutVarint writing to buf[0:] on every iteration instead of advancing the write position
• Each validator's encoded priority now occupies its own position in the buffer
• Add test verifying that changing a single validator's priority produces a different hash

Test plan

• go vet ./sei-tendermint/types/... passes
• go test ./sei-tendermint/types/... passes
• New test fails without the fix, passes with it

[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	Apr 15, 2026, 10:12 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.25%. Comparing base (de50195) to head (8a7bc96).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3255      +/-   ##
==========================================
- Coverage   59.28%   59.25%   -0.03%     
==========================================
  Files        2070     2070              
  Lines      169775   169704      -71     
==========================================
- Hits       100655   100563      -92     
- Misses      60328    60340      +12     
- Partials     8792     8801       +9     

Flag	Coverage Δ
sei-chain-pr	81.62% <100.00%> (?)
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-tendermint/types/validator_set.go	93.10% <100.00%> (ø)

... and 28 files with indirect coverage changes

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