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

`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:

```yaml
- 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.