fix: lthash worker loop break; remove unreachable digest.Read fallback

[blindchaser]

Describe your changes and provide context

• Since start := w * chunkSize is monotonic, we switched both loops from continue to break to avoid unnecessary iterations.
• remove unreachable digest.Read fallback

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	Jan 14, 2026, 9:23 PM

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.82%. Comparing base (7a3b6b4) to head (eb08c9d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/flatkv/lthash/api.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2698   +/-   ##
=======================================
  Coverage   43.82%   43.82%           
=======================================
  Files        1908     1908           
  Lines      158967   158948   -19     
=======================================
  Hits        69664    69664           
+ Misses      82904    82885   -19     
  Partials     6399     6399           

Flag	Coverage Δ
sei-chain	45.91% <33.33%> (+0.01%)	⬆️
sei-cosmos	38.20% <ø> (+<0.01%)	⬆️
sei-db	69.43% <ø> (ø)
sei-tendermint	47.29% <ø> (-0.02%)	⬇️

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/flatkv/lthash/lthash.go	91.30% <100.00%> (+12.94%)	⬆️
sei-db/state_db/sc/flatkv/lthash/api.go	85.71% <0.00%> (ø)

... and 15 files with indirect coverage changes

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