feat(accumulator): min/max/product aggregate operations (A2)

[discreteds]

A2 — aggregate operations beyond sum

Extends AccumulatorEngine to accumulate min/max/product numerics in addition to sum, via a new AggregateOp StrEnum validated at model construction.

What changed

• AggregateOp(StrEnum) (sum/min/max/product — commutative reducers only; replace/coalesce are explicit non-goals) in engines/accumulator/aggregate.py; exported from the package root.
• Aggregate.operation retyped str → AggregateOp — invalid operations now raise ValidationError at construction (fail-fast) rather than ValueError at build.
• Fold dispatch in engine._expand_level: SUM→add, MIN→ma.least, MAX→ma.greatest, PRODUCT→mul — all backend-pure (no new native imports, no new # allow: tags).
• CLAUDE.md updated.

Provenance

Split out of the A1+A2 accumulator-core-extensions work (spec docs/superpowers/specs/2026-07-19-accumulator-core-extensions-design.md). A2 is independent and lands on its own; A1 (set coalescing) is deferred — a whole-branch review found the null-list wildcard representation breaks the frontier self-join across backends, so A1 is being redesigned around in-band sentinels (see mountainash-central principle null-is-not-a-portable-sentinel).

Tests

• tests/accumulator/test_aggregate.py — enum + model validation.
• tests/accumulator/test_engine.py::TestAggregateOperations — min/max/product/sum folds over a {R1,R2} combination (prime products 2/3/6).
• tests/accumulator/test_edge_cases.py — unsupported-op now asserts construction-time ValidationError.
• Full quick suite: 812 passed, 36 skipped, 31 xfailed; ruff + backend-purity green.

🤖 Generated with Claude Code