feat(layout): add the ExecNode trait and push-based exec graph

Add `plan::exec`, a push-based executor interface for physical plans.

`ExecNode` is the per-operator trait: a single `compute` that takes the
events that arrived for the node (child pieces, child closes, segment
deliveries) through a `StepCx`, and spawns children, requests segments,
and emits row-tagged `Piece`s through it. It reports `Wait`, `Yield`, or
`Done`.

`ExecGraph` is the flat node arena. It wires nodes child-to-parent, routes
IO deliveries to the leaves that requested them, and hands its owner the
root's pieces and batches of `IoRequest`s from `compute`. Nodes never talk
to the IO service; the owner answers requests with `set_io_result`.

`PlanVTable::exec` builds the node for an operator, reached through
`PlanRef::exec`. No operator implements it yet; the default bails.

Signed-off-by: Joe Isaacs <joe.isaacs@live.co.uk>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NWYmfHTFtaEdhdyd3u5hv3

Author: joseph-isaacs

Cargo.lock

@@ -35,6 +35,7 @@ pin-project-lite = { workspace = true }
 prost = { workspace = true }
 rustc-hash = { workspace = true }
 sketches-ddsketch = { workspace = true }
+smallvec = { workspace = true }
 termtree = { workspace = true }
 tokio = { workspace = true, features = ["rt"], optional = true }
 tracing = { workspace = true }