feat(types): canonical H2O column wrappers — top/bot/pearson_corr/std/__pow__

[ser-vasilich]

Summary

Single commit (`c0d315c`) adding new Column methods needed for canonical H2O groupby coverage in rayforce-bench. 56 lines across 3 files — pure Python AST additions, no C/FFI changes.

New methods on `Column` / `AggregationMixin`

• `Column("v3").top(n)` — n largest values per group, descending (canonical H2O q8)
• `Column("v3").bot(n)` — n smallest values per group, ascending
• `Column("v1").pearson_corr(Column("v2"))` — Pearson correlation (q9, with `** 2` second stage)
• `Column("v3").std()` — sample stddev, ddof=1, matches polars/pandas (q6 stddev side)
• `Column("v3") ** 2` (`pow` / `rpow`)

New `Operation` enum entries

`POW`, `STDDEV`, `PEARSON_CORR`, `TOP`, `BOT`.
`MEDIAN` and `DEVIATION` already existed (engine verbs `med` / `dev`).

Each wrapper is a 1-line `return Expression(Operation.X, self, …)` — relies only on the existing Expression-AST mechanism, no v2-specific dependencies.

Engine support

Verb names match engine opcodes landing in:

• feat: canonical H2O groundwork — top/bot/pearson_corr/pow + 4 fixes rayforce#202 (canonical H2O groundwork — `top`, `bot`, `pearson_corr`, `pow` primitives)
• perf: canonical H2O q6/q8/q9 — OP_MEDIAN, OP_PEARSON_CORR, OP_TOP_N, OP_GROUP_TOPK_ROWFORM rayforce#203 (perf opcodes — OP_MEDIAN, OP_PEARSON_CORR, OP_TOP_N, OP_GROUP_TOPK_ROWFORM)

This PR is the Python surface; without the engine PRs above, the new methods compile but error at runtime with the engine's missing-op message.

Related

• Earlier PR feat: v2 finalization + canonical H2O column wrappers #10 was closed — it bundled 42 commits of Karim's pre-existing v2 migration work alongside this single wrapper commit. Scope was wrong; v2 migration is independent work that should land separately. This re-PR is just our 1 commit on top of `master`.

Test plan

• Cherry-picks cleanly onto `origin/master` (1.0.0 base) — no v2 dependency
• Reviewer: against engine PR #202 + #203 checkout — `make check LOCAL=1` in rayforce-bench → `pass — 665/665`

[ser-vasilich]

Closing — wrong base. Wrapper depends on v2 binding (feat/canonical-h2o-wrappers branch), but this PR targets master (v1.0.0). Will re-open against v2 once that branch is on origin.