Add focused property-based tests for import, chunking, export, and dedup invariants

[edheltzel]

Goal

Use property-based tests where they prove invariants that example tests miss, without adding test theater.

Dependencies

• Hard dependency: Audit SQLite variable limits and add shared chunking #41 for the shared SQLite chunking helper.
• Export properties depend on Add mem export with JSON, Markdown, SQL dump, and SQLite backup formats #43.
• Dedup properties depend on Add non-destructive dedup with provenance-aware survivor selection #45.
• If this issue is implemented as one ticket, do not mark it complete until Add mem export with JSON, Markdown, SQL dump, and SQLite backup formats #43 and Add non-destructive dedup with provenance-aware survivor selection #45 exist. If work starts earlier, complete only the currently unblocked property groups and leave the remaining groups explicitly pending in this issue.

Acceptance criteria

• Add fast-check or an equivalent Bun-compatible property-test library using the project’s existing dependency style.
• All property tests run under bun test.
• Conversation import normalization properties cover:
  • required fields are preserved/derived correctly.
  • invalid and empty inputs are rejected deterministically.
  • the same input produces deterministic normalized output.
• SQLite chunking properties cover:
  • order preservation.
  • exactly-once processing.
  • boundary sizes 0, 1, 499, 500, 501, 999, 1000, 1001, and larger N.
• Export properties, once Add mem export with JSON, Markdown, SQL dump, and SQLite backup formats #43 exists, cover:
  • JSON structural stability for generated records.
  • Markdown structural stability for generated records.
  • provenance presence for provenance-bearing exported tables.
  • manifest counts match exported records.
• Dedup properties, once Add non-destructive dedup with provenance-aware survivor selection #45 exists, cover:
  • idempotence.
  • survivor priority by provenance.
  • non-duplicate preservation.
  • duplicate lineage remains auditable after repeated runs.
• Properties should test invariants, not current default strings or formatting details.
• Keep generated test sizes bounded so the suite remains practical under normal bun test runs.

[edheltzel]

PR #57 implements the currently unblocked property groups (conversation import normalization + SQLite chunking) with fast-check under bun:test. Per the dependency note in this issue, the remaining groups stay explicitly pending:

• Export properties — blocked on Add mem export with JSON, Markdown, SQL dump, and SQLite backup formats #43
• Dedup properties — blocked on Add non-destructive dedup with provenance-aware survivor selection #45

If #57 merges via Closes #44, reopen or re-track those two groups once #43/#45 land.