M6d: tuple-stream group-by — the reduce half of joins

[flearc]

Summary

Ports JSONata v2.2.1's reduceTupleStream so the {…} group-by operator propagates tuple-stream bindings ($e/$c/$i from @$v/#$v) into its key/value expressions — the "reduce" half of map-reduce joins (M6c delivered the "map" half).

• Evaluator (c3e3c84): two helpers — tuple_append (jsonata fn.append) and reduce_tuple_stream (merge a group's tuples, appending each binding so $c becomes every contact in the group; a 1-member group keeps the scalar). eval_group_step gains a tuple_mode: the key is evaluated with a per-tuple frame; the value is evaluated against the merged tuple's @ (group context) under a frame binding the merged members (so both $join($c.Phone.number) and bare Phone.number value shapes work). The eval_path_tuple group branch passes tuples through instead of collapsing to @-values. The non-tuple group-by path is byte-identical.
• Baseline (1930760): official-suite regen, zero regressions.

Results

• Official suite 1237 → 1246 (+9); joins group 27 → 36/43 (employee-map-reduce 1–6,9,10 + the index group-by case).
• 488 unit tests green; zero-regression guard green (group-by/transform/sorting byte-identical — eval_group_step's non-tuple path is unchanged).
• Adversarial review vs genuine jsonata-js v2.2.1: no in-scope divergences. Verified multi/single-member reduce, context-vs-binding value shapes, value-side ^($) sort, index group-by, D1009/T1003. On the empty-tuple-stream edge we return {} while jsonata-js throws an uncaught TypeError (upstream bug) — we keep the graceful result.

Deferred (the remaining joins gaps)

• Cases 7,8 — sort-on-a-focus-step (Employee@$e^($e.Surname)…): a separate sort-tuple-stream bug (we return null, no crash).
• M6c reorder cases — filter/sort before # (ordered-stages).

Test plan

• busted spec/ — 488/0
• busted spec/jsonata_suite_spec.lua — zero-regression guard green
• bash scripts/run-suite.sh — 1246/1682, joins 36/43
• Adversarial oracle fidelity review (jsonata@2.2.1)

🤖 Generated with Claude Code