feat(expr-ir): Add concat

[dangotbanned]

Description

Episode 5 is aiming for DslPlan::Union and DslPlan::HConcat.

narwhals/narwhals/_plan/logical_plan/plan.py

Lines 300 to 308 in 63dbc65

	# `DslPlan::Union`
	class VConcat(MultipleInputs[Seq[LogicalPlan]]):
	"""`concat(how= "vertical" | "diagonal")`."""
	__slots__ = ("options",)
	options: VConcatOptions
	def __repr__(self) -> str:
	return "UNION"

narwhals/narwhals/_plan/logical_plan/plan.py

Lines 311 to 319 in 63dbc65

	class HConcat(MultipleInputs[Seq[LogicalPlan]]):
	"""`concat(how="horizontal")`."""
	__slots__ = ("strict",)
	strict: bool
	"""Require all `inputs` to be the same height, raising an error if not, default False."""
	def __repr__(self) -> str:
	return "HCONCAT"

---

Back over here, the compliant-level was already there - just need to get the tests in place now.

Related issues

• Child of feat(RFC): A richer Expr IR #2572
• Follows feat(expr-ir): Add read_{csv,parquet} #3370
• Towards oh-nodes...expr-ir/logical-plan