V6: Diff / compliance engine

[ckrough]

Complexity: L

Scope

The diff / compliance engine. Given the source-of-truth Composition snapshot (from #304's resolver) and each channel/hop's ListingSnapshot, compute:

• Per-hop and per-channel status: in_sync / stale / missing.
• The version delta (live version stamp vs. current Composition version).
• A normalized text diff for stale rows.

Persist the resulting sync-status rows via #304's model.

Dependencies

• Blocked by: V2: Compliance data model + source-of-truth resolution + Alembic + org RLS #304 (schema, resolver, match function), V3: Channel-adapter interface + Petfinder API v2 read adapter #305, V4: Adopt-a-Pet feed/API read adapter #306, V5: Composition -> SMS drift check via Pet Data #307 (the snapshots to compare). The engine can be built and tested against fixture snapshots before the externally-blocked adapters are live.
• Blocks: V7: Scheduled + on-demand compliance runs #309, V8: Typed, authed, org-scoped API serving the compliance table #310.

Acceptance criteria

• Identical normalized text and a matching version stamp -> status in_sync (test).
• Text differing by a word, or an older live version stamp -> status stale, with the diff populated (test).
• A missing snapshot -> status missing (test).
• Running the engine over a fixture batch persists one status row per animal x channel x hop, and they read back through the V2: Compliance data model + source-of-truth resolution + Alembic + org RLS #304 store (integration test).

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ADR: docs/adr/0033-listing-sync-verification.md. Epic: #302.