Plan-graph round 8 (owner): placement now runs on dagre — mermaid's engine

The owner chose: adopt the same layout engine regular mermaid uses, keep
our SVG skin. That engine is dagre (Sugiyama-style layered layout) — and
mermaid 11 ships dagre-d3-es 7.0.14, so we import exactly that fork, via
the same deep subpath mermaid itself uses. No new code shipped; pinned
verbatim in package.json (lane touch — flagged to master).

blockLayout keeps its signature ({ x, y, nodeH, width, height }, all
consumers unchanged): it feeds dagre our geometry (NODE_W boxes, GAP_X/
GAP_Y separations, zero margin, per-node grown heights from round 7),
runs the layout (deterministic, synchronous, DOM-free — SSR and render
tests stay exact), and converts dagre's node centers back to top-lefts.
Dangling edges are ignored defensively (a ghost endpoint would spawn an
unmeasured node and break the measurer).

topoFor stays exported for order/depth semantics (tests); placement no
longer calls it. dagre's network-simplex ranker + crossing minimization
now handle multi-parent DAGs, so future plan shapes route like mermaid
diagrams do. On the existing tree fixtures dagre's numbers agree exactly
with the old hand-rolled layout — every render pin survives verbatim.

package.json

@@ -29,6 +29,7 @@
 		"class-variance-authority": "^0.7.1",
 		"clsx": "^2.1.1",
 		"cmdk": "^1.1.1",
+		"dagre-d3-es": "7.0.14",
 		"embla-carousel-react": "^8.6.0",
 		"graphology": "^0.26.0",
 		"graphology-layout-forceatlas2": "^0.10.1",