docs: add Halborn 2026-08 audit report

[Maikol]

Adds the Halborn assurance assessment (Aug 4 - Aug 31, 2026) of the Horizon, subgraph-service, issuance and rewards/curation contracts at commit 78d3716.

• Added as packages/horizon/audits/2026-08-Halborn-contracts.pdf (re-saved with ghostscript to reduce size, content unchanged).
• This is the pre-remediation version: all findings are listed as pending. It will be replaced with the final report once our responses are incorporated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.47%. Comparing base (3488faa) to head (ba1919a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   91.47%   91.47%           
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  Files          81       81           
  Lines        5326     5326           
  Branches      973      973           
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  Hits         4872     4872           
  Misses        433      433           
  Partials       21       21           

Flag	Coverage Δ
unittests	91.47% <ø> (ø)

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