`foreach:` is not a for_each alias — no engine expands it

[jeremymanning]

core/loop_contracts.py (merged in #473) declared foreach a supported alias of for_each, on the strength of a comment rather than a run. It is not one.

Two shapes, run through the hermetic Python API:

- id: loop
  foreach: "['A','B']"
  steps:
    - id: w
      tool: filesystem
      action: write
      parameters: {path: "out/{{ index }}.txt", content: "{{ item }}"}

→ YAMLCompilerError: Failed to compile pipeline: Schema validation failed: 325 errors

The same file with for_each: writes 0.txt and 1.txt containing A and B.

In a shape that does compile, the body runs once and $item/$index resolve to None — the loop never expands.

Where the spelling does appear:

• engine/pipeline_spec.py:72 converts a foreach field into a LoopSpec, for the declarative engine only.
• compiler/control_flow_compiler.py:119 — the principal compiler — branches on for_each only, and _create_for_each_task reads only that spelling.

So the two engines disagree, and the validator was siding with the one that does not run the pipeline. #479 removes foreach from the loop contracts so validation stops reporting it as a working construct.

The remaining decision is whether foreach should be implemented in the control-flow compiler or rejected at schema level. It is currently neither: it parses and silently does nothing. Catalogue files that use it: examples/migration_showcase.yaml, examples/web_research_pipeline.yaml, examples/advanced/parallel_processing.yaml — all currently failing validation for other reasons too.