refactor(snapshot-legacy-importer): clean up the importer and add unit tests

[oXtxNt9U]

Summary

Changes

• Snapshots are read only as brotli (<hash>.compressed). The unused .json path is removed, read failures name the file, and the console output is gone.
• Removed the total supply check in import(). It compared the prepared wallet list against itself, so it could never fail.
• Removed unused API: LegacyImporter.totalSupply and .result, arkAddress/publicKey on imported voters and validators, and the empty defaults.ts.
• import() now rejects an import before prepare() or a second import, and dispose() resets all state.
• A failed contract call during import names the validator, username or vote batch, and includes the decoded revert reason.
• run() errors now name what didn't match: snapshot hash vs. milestone, chain tip vs. genesis parent hash, or a missing snapshot milestone.
• bootstrap: fixed the reversed error message for a snapshot milestone with a zero genesis parent hash.

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.80%. Comparing base (668d31b) to head (42ed2ac).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1525      +/-   ##
===========================================
+ Coverage    80.39%   80.80%   +0.40%     
===========================================
  Files          952      962      +10     
  Lines        17178    17663     +485     
  Branches      2611     2739     +128     
===========================================
+ Hits         13810    14272     +462     
- Misses        3363     3383      +20     
- Partials         5        8       +3     

Flag	Coverage Δ
contracts	91.95% <ø> (?)
packages	80.52% <100.00%> (+0.12%)	⬆️

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