Bound memory for exact-only freezes

[biru-codeastromer]

Summary

• stream strict JSONL, CSV, and optional Parquet records through disk-backed digest spools
• preserve canonical manifest and public attestation bytes while committing and encrypting incrementally
• retain the in-memory path for similarity plugins
• add byte-equivalence, bounded-memory, and local benchmark coverage

This replaces #23 with one linear commit based on the merged #22 head. The resulting content tree is exactly identical to #23 while excluding its superseded merge ancestry.

Equivalence

• parent: e97c0b7
• tree: a542307b3c6d325dc9bad614532d626a573cfe03
• source and replacement patch ID: c87db778f85f73b33294cd3acbfa59f60768be46
• author and committer use the generic repository identity

Validation

• local make release-gate: 132 passed, 1 optional skip, 92.40% coverage
• formatting, Ruff, mypy, clean 0.4.0 installation, dependency audit, examples, repository and text audits passed
• full-history Gitleaks passed across 86 commits

This PR requires a fresh eligible approval before merge.