feat: Simple ConvexChecker for one-off checks.

[aborgna-q]

TopoConvexChecker does a full graph traversal on initialization, to optimise online is_convex queries.
This incurs an extra cost that is not amortized when running a single check before discarding the structure.

See for example

portgraph/src/view/subgraph.rs

Lines 99 to 103 in ebaed87

	/// Whether the subgraph is convex.
	pub fn is_convex(&self) -> bool {
	let checker = TopoConvexChecker::new(self.graph());
	self.is_convex_with_checker(&checker)
	}

We should implement a simpler convexity checker for single operations, and use it where appropriate.

Discussion: CQCL/hugr#1656
Note the optimization proposal for only exploring nodes in the same hierarchy region. That could be used both here and in the general TopoConvexChecker.