Priority Level: High (Major functionality broken)

Describe the bug

The async engine (DATA_DESIGNER_ASYNC_ENGINE=1) has two issues with side-effect columns from @custom_column_generator:

1. Side-effect values not propagated to buffer - AsyncTaskScheduler._run_cell() writes results back via _instance_to_columns, which only contains primary column names. Side-effect values (e.g., _tag_notation, _merged_tagged_text) are in the result dict but get filtered out, so downstream prompts that reference them via Jinja2 fail with "columns are missing".

2. Side-effect collision silently misresolves - ExecutionGraph.set_side_effect() uses last-writer-wins. When two generators declare the same side-effect column at different pipeline stages, the second registration overwrites the first, creating incorrect dependency edges and false DAGCircularDependencyError.

This blocks Anonymizer from using the async engine (NVIDIA-NeMo/Anonymizer#95).

Steps/Code to reproduce bug

# Two custom columns producing the same side-effect at different stages
@custom_column_generator(
    required_columns=["seed_col"],
    side_effect_columns=["shared_side_effect"],
)
def stage_1(row):
    row["stage_1_col"] = "value"
    row["shared_side_effect"] = "produced_by_stage_1"
    return row

@custom_column_generator(
    required_columns=["seed_col", "stage_2_col"],
    side_effect_columns=["shared_side_effect"],
)
def stage_3(row):
    row["stage_3_col"] = "value"
    row["shared_side_effect"] = "produced_by_stage_3"
    return row

# LLM column between them references {{ shared_side_effect }} in prompt.
# Graph resolves shared_side_effect -> stage_3 (last writer), not stage_1.
# Creates false cycle: stage_3 -> stage_2 -> stage_3

With DATA_DESIGNER_ASYNC_ENGINE=1, Anonymizer's detection pipeline (10 generated columns, multiple stages with side-effect reuse) hits:

DAGCircularDependencyError: The execution graph contains cyclic dependencies. Resolved 6/12 columns.

After manually breaking the cycle by removing later side-effect declarations, a second error surfaces:

The following ['_tag_notation', '_merged_tagged_text'] columns are missing!

Expected behavior

1. ExecutionGraph.set_side_effect() should detect collisions and keep the first registration (matching sync engine semantics where earlier consumers see earlier producers), or raise.
2. Side-effect column values from cell task results should be written to the RowGroupBufferManager so downstream tasks can read them via prompt templates.

Agent Diagnostic / Prior Investigation

Issue 1 root cause: async_scheduler.py line 128-131 builds _instance_to_columns from generators.items() (primary columns only). At line 785, the write-back loop for col in output_cols skips side-effect columns even though they're present in the result dict. Fix: include config.side_effect_columns when building _instance_to_columns.

Issue 2 root cause: execution_graph.py line 108, set_side_effect is a blind dict assignment. In Anonymizer's detection pipeline, prepare_validation_inputs and merge_and_build_candidates both produce _merged_tagged_text. The second overwrites the first, so _validation_decisions's prompt reference {{ _merged_tagged_text }} resolves to _merged_entities instead of _validation_candidates, creating the cycle: _merged_entities -> _validation_decisions -> _validated_entities -> _seed_entities_json -> _augmented_entities -> _merged_entities.

Additional context

The sync engine handles both cases correctly because columns execute sequentially on a shared mutable row dict. Both fixes are small and localized (~5 lines each). Workaround: DATA_DESIGNER_ASYNC_ENGINE=0 (the default).