feat: Lattice.save/load — accumulator snapshot persistence

[discreteds]

First-class build-offline/serve-online persistence for accumulator lattices.

What

• Lattice.save(dir) writes a snapshot directory: lattice.parquet (combinations frame verbatim — co_*, __agg_*, __prime_product, __level all preserved) + manifest.yaml sidecar (dimensions, aggregates, partition_key).
• Lattice.load(dir) rehydrates through the public constructor; is_composed round-trips honestly (__prime_product travels in the parquet).

Why

Build-offline → serve-online had no first-class persistence — consumers (mountainash-rules-service LatticeRegistry) hand-rolled parquet + sidecar. Babel CSV can't serve accumulated values (its importer strips __agg_* by contract). This retires the hand-rolled loader.

Design notes

• Backend purity preserved. save stays pure (relation(...).to_polars().write_parquet(...)). load carries the package's third # allow:-tagged import polars (snapshot parquet read), documented in CLAUDE.md.
• Manifest is a strict superset of babel's LatticeManifest — dimensions/aggregates shapes are byte-compatible, so babel/service tooling reads rules-written snapshots unchanged (pydantic ignores the extra partition_key). One-way by design; documented in the spec.

Tests

5 new tests in tests/accumulator/test_lattice.py: round-trip identity, apply-equivalence (accumulated values compared), file creation, manifest-superset shape, missing-manifest FileNotFoundError. Full quick suite 758 passed / 36 skipped / 31 xfailed; ruff clean; purity gate green.

Spec: docs/superpowers/specs/2026-07-16-lattice-snapshot-save-load-design.md · Plan: docs/superpowers/plans/2026-07-16-lattice-snapshot-save-load.md

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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