The layout vocabulary's biggest gap, after D2/TALA's 'containers are
first-class at every layout stage': hierarchical zones and arrow-ordered
graphs could not mix. draw_graph did whole-page flowcharts, containers did
nesting, and 'an architecture zone whose contents follow the data flow' was
inexpressible.
add_graph is a macro operation: nodes+edges go through the existing layered
pass (graph.ts — cycle breaking, longest-path layering, barycentre crossing
reduction) which emits ordinary container/box/link operations, and the
resulting block participates in the outer flexbox like any node. dir col/row
transposes the flow. No new layout code — the coordinate work was always
generic; what was missing was the entry point below page level.
Synthetic layer ids are namespaced by the graph's own id (g1__layer0), fixing
the collision that previously made a second graph per page impossible.
graph.ts gains parent/prefix/rootId options; draw_graph keeps its behaviour
as the page-level case of the same code path.
4 new tests: embedding in a flexbox column, two graphs per page, dir
transposition, unknown-endpoint errors. 565 unit tests green, 8 engine e2e
green. Acceptance: three-zone architecture diagram (person/cloud zone,
arrow-ordered pipeline zone with decision branches and a bold arrow,
cylinder/queue storage zone, cross-zone links) verified in the real editor.
