Normalize excessive structured input nesting

[biru-codeastromer]

Summary

• cap structured JSON inputs at 100 container levels with the same boundary for scalar and empty-container leaves
• normalize recursion failures across canonicalization, JSONL, TOML, artifact parsing, and authenticated private-seal plaintext
• preserve SS011 for structured values, SS040 for malformed decrypted manifests, and SS044 plus JSON stderr for malformed artifacts

Validation

• make release-gate
• 100 passed, 1 optional skip
• 91.80% coverage
• exact 100-level acceptance plus 101-level scalar and empty-container rejection
• authenticated plaintext regressions at 101 and 2,000 levels return SS040
• clean wheel and source installation
• dependency, text, repository, example, and full-history Gitleaks audits passed

This PR is intentionally left unmerged pending maintainer review.

[biru-codeastromer]

Superseded by merged #41, which preserves this change in the protected v0.4.0 release stack.