feat(diagram-engine): wire up restructure_diagram + stencil catalog

Closes the loop: the model can now build and edit AWS architecture diagrams by
declaring structure, and never writes an mxCell again.

catalog.ts — 983 AWS icon and 19 group stencils as a name→style map, generated from
drawio-ai-kit's catalog (itself generated from jgraph's draw.io shape index). Styles
are verbatim, so the official category colours, connection points and aspect=fixed
come along for free and nothing is hand-assembled. An invented name is rejected with
suggestions instead of rendering as a blank square, which is what draw.io does with
an unknown resIcon today.

operations.ts — what the model actually sends: add_icon / add_container / move /
link / set_dir and so on, applied in order against the tree. Guards the things that
break a diagram quietly: duplicate ids (draw.io drops one of the two cells), edges
left pointing at a removed node, and moving a container inside itself.

index.ts — the entry point. current XML → parse → apply ops → check names → layout →
render → new XML. The tree is not stored between calls; it is re-derived from the
canvas every time, so a user's manual edits are input to the next layout rather than
state to reconcile.

Token cost, measured with Claude's tokenizer rather than estimated:
  - build a VPC diagram:  515 tok as operations vs 3180 as XML   (6.2x)
  - add one icon:          27 tok as an operation vs 3823 re-emitting (142x)
  - read current state:   216 tok as an outline vs 3180 as XML   (14.7x)

The 142x is the one that matters day to day: "add a Redis" is one operation, not a
rewrite of the whole diagram.

Routing in the system prompt sends AWS architecture through this path and leaves
flowcharts, BPMN, sequence diagrams, mind maps and Azure/GCP on display_diagram —
the layout engine's primitives (nested rows, columns, grids) do not model a sequence
diagram's lifelines or a mind map's radial spread, and pretending otherwise would
make those worse rather than better.

Also: added a NOTICE recording the MIT port and the AWS Architecture Icons terms,
and a narrow .gitignore exception so the generated catalog is tracked while the
root data/ directory (admin settings, contains secrets) stays ignored.

403 unit tests + 3 new e2e. Verified in the real app: a structural tool call renders
with real stencils and container markers; a second call adds one node and keeps
everything from the first; an invented name is refused and nothing is drawn. The 13
existing diagram e2e tests still pass.

NOTICE

@@ -0,0 +1,43 @@
+next-ai-draw-io
+Copyright the next-ai-draw-io contributors
+
+This product includes software developed by third parties, as set out below.
+
+--------------------------------------------------------------------------------
+lib/diagram-engine/ — layout and rendering
+--------------------------------------------------------------------------------
+
+The declarative layout algorithm (bottom-up measure, top-down place, sibling
+size equalisation) and the mxCell/style emission in `lib/diagram-engine/layout.ts`
+and `lib/diagram-engine/render.ts` are derived from drawio-ai-kit:
+
+    https://github.com/sparklabx/drawio-ai-kit
+    Copyright (c) sparklabx
+    Licensed under the MIT License
+
+The XML→tree reverse parser (`lib/diagram-engine/parse.ts`), the style-marker
+scheme (`markers.ts`), the structural-operations layer (`operations.ts`) and the
+invisible-container approach that replaces that project's "phantom" nodes are
+original to this repository.
+
+--------------------------------------------------------------------------------
+lib/diagram-engine/data/aws-stencils.json — stencil catalog
+--------------------------------------------------------------------------------
+
+A name→style map for the mxgraph.aws4 stencil family, generated from
+drawio-ai-kit's `catalog/aws.json`, which in turn was generated from the draw.io
+shape index published by jgraph:
+
+    https://github.com/jgraph/drawio-mcp
+    Copyright (c) JGraph Ltd
+    Licensed under the Apache License, Version 2.0
+
+The style strings are reproduced verbatim from that index. They reference the
+official AWS Architecture Icons, which are trademarks of Amazon Web Services and
+are NOT covered by this repository's licence. Their use is governed by the AWS
+Architecture Icons terms:
+
+    https://aws.amazon.com/architecture/icons/
+
+The catalog contains stencil *names and style strings* only — it does not embed
+any AWS icon artwork. draw.io supplies the artwork at render time.