Add `reframe()` verb

[maciekbanas]

Summary

Implement reframe.tbl_lazy_json. Like summarise() but allows results of arbitrary length per group (no one-row-per-group restriction).

Requirements

• Reuse the grouping machinery.
• Generalise the summarise.js handler so a group can emit multiple rows (aggregation may produce a vector).
• Support the .by argument (see the .by tracking work).

Acceptance criteria

• reframe.tbl_lazy_json exported and re-exported from dplyr in NAMESPACE.
• Multi-row-per-group output supported.
• summarise.js generalised without breaking summarise().
• Unit tests for lazy-step accumulation; shinytest2 integration for multi-row output.
• Roxygen docs; devtools::document() regenerated.

Part of the jsplyr 0.2.0 release. See spec.md.