fix(flatkv): prevent phantom MixOut for new accounts in LtHash

[blindchaser]

Describe your changes and provide context

• When a new account (not in accountDB) was first written, getAccountValue returned a zero AccountValue{}. Encoding it produced 40 zero-bytes (len>0), so LtHash performed a MixOut on data that was never MixIn'd, corrupting the hash.
• Replace oldAccountValues (map[string]AccountValue) with oldAccountRawValues (map[string][]byte) that stores raw DB bytes. Accounts not found in DB get nil, and LtHash correctly skips MixOut when len(LastValue)==0.

Testing performed to validate your change

• Add lthash_correctness_test.go with 100-block end-to-end test verifying incremental LtHash matches full-scan at every checkpoint.
• Add focused regression tests for new-account phantom MixOut and multi-ApplyChangeSets-per-block scenarios

[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	Mar 2, 2026, 11:23 PM

[codecov]

Codecov Report

❌ Patch coverage is 82.60870% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.30%. Comparing base (778ced7) to head (ccee95d).

Files with missing lines	Patch %	Lines
sei-db/state_db/sc/flatkv/lthash/lthash.go	50.00%	1 Missing and 1 partial ⚠️
sei-db/state_db/sc/flatkv/store_write.go	89.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3000      +/-   ##
==========================================
- Coverage   58.30%   58.30%   -0.01%     
==========================================
  Files        2108     2108              
  Lines      173668   173648      -20     
==========================================
- Hits       101259   101238      -21     
- Misses      63389    63392       +3     
+ Partials     9020     9018       -2     

Flag	Coverage Δ
sei-chain-pr	66.08% <82.60%> (?)
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-db/state_db/sc/flatkv/lthash/lthash.go	89.91% <50.00%> (-1.39%)	⬇️
sei-db/state_db/sc/flatkv/store_write.go	70.16% <89.47%> (+1.41%)	⬆️

... and 34 files with indirect coverage changes

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