feat(diagram-engine): open shape vocabulary — catalog + pass-through, structured style merge

The expressiveness gap traced to vocabulary: draw.io has hundreds of shapes,
the declarative layer allowed six. Opening it, with the failure modes from
design review handled:

- shapes.ts: a ~20-entry curated catalog (full style fragment incl. the
  matching perimeter — required or edges connect to the bounding box; a
  text-scale factor verified in the real editor — the same sentence overflows
  a 1.0x rhombus and fits a 1.5x one; labelOutside+glyph for umlActor-style
  figures). Any other token passes through verbatim: draw.io degrades unknown
  shapes to rectangles safely (verified). Injection-capable tokens (;/=) are
  rejected outright.
- Pass-through emits a WARNING with a nearest-catalog hint (bounded edit
  distance), so a typo'd 'cyclinder' is a one-turn fix instead of a silently
  rectangular node forever. set_shape/set_role/set_group operations make the
  fix possible without remove+re-add (which would drop links).
- mergeStyle(): style fragments merge per-key (later wins, bare shape classes
  displace each other) instead of string concatenation. This is what makes
  shape and theme composable by rule — shape owns geometry keys, theme owns
  colour/type keys, and an overlap resolves by order instead of emitting
  contradictory duplicates.
- dai_shape marker carries the declared token through the round trip:
  appearance-based reverse mapping cannot distinguish aliases (diamond vs
  decision) or a rotated queue from a cylinder.
- dai_auto marker separates engine-measured size from user-fixed size: parsed
  boxes no longer freeze the first layout's numbers, so changing a label
  re-measures. Pinned nodes keep everything, as before.
- draw_graph's closed shape enum opened to match (first instance of the
  schema-drift problem the review predicted).

14 new tests: merge ownership, injection rejection, near-match hints,
alias-preserving round trip, re-measure on label change, mxgraph.* tokens
staying boxes with role/group intact. 556 unit tests green; acceptance
diagram (person/hexagon/cylinder/queue/cloud/decision/callout) verified in
the real editor.

app/api/chat/route.ts

@@ -790,7 +790,7 @@ Loops are fine — an arrow back to an earlier step is drawn as a loop. So are a
 
 Replaces the whole diagram, because one new arrow can change which row several nodes belong in. To edit afterwards, use restructure_diagram with the ids from the outline this returns.
 
-Shapes: "decision" for a branch (diamond), "terminator" for a start or end point, "data" for input or output, "document" for a report, "round" for a soft-edged step, "box" (default) for a plain step. Set icon instead of shape to draw a node as a catalog icon — look the name up with search_stencils first.
+Shapes say what a node IS: "decision" for a branch (diamond), "terminator" for a start or end point, "data" for input or output, "document" for a report, "round" for a soft-edged step, "cylinder" for a database, "queue" for a message queue, "person" for an actor or user, "cloud" for an external system, "hexagon" for a service, "ellipse" for a concept, "box" (default) for a plain step. Any other draw.io shape token also works verbatim. Set icon instead of shape to draw a node as a catalog icon — look the name up with search_stencils first.
 
 Grouping: when the nodes fall into natural zones (remote vs local, frontend vs backend, roles, phases), set the same group name on each zone's nodes. The engine colours each group consistently from its own palette. Name groups by meaning; never pick hex colours.`,
                 inputSchema: z.object({
@@ -800,15 +800,11 @@ Grouping: when the nodes fall into natural zones (remote vs local, frontend vs b
                                 id: z.string(),
                                 label: z.string(),
                                 shape: z
-                                    .enum([
-                                        "box",
-                                        "decision",
-                                        "terminator",
-                                        "round",
-                                        "data",
-                                        "document",
-                                    ])
-                                    .optional(),
+                                    .string()
+                                    .optional()
+                                    .describe(
+                                        "What the node IS: decision, terminator, round, data, document, cylinder (database), queue, person (actor), cloud (external), hexagon (service), ellipse. Any draw.io shape token also works",
+                                    ),
                                 icon: z
                                     .string()
                                     .optional()