feat(diagram-engine): flex knobs (grow/align/pad) + inline rich-text labels

The expressiveness gap between engine output and hand-written XML came down
to two missing capabilities, both generic:

- Block layout inside a box: nested containers already existed, but there was
  no way to split space by weight, pin a child to an edge, or tighten padding.
  Added grow (flex-grow over the parent's leftover flow-axis space, TeX's
  glue), align (start/center/end/stretch on the cross axis) and pad
  (per-group interior padding). All three round-trip via dai_grow/dai_align/
  dai_pad markers.

- Inline rich text: labels already render HTML (html=1 on every style, esc()
  entities decode back), but the measure pass counted markup as text. The
  visibleText() strip makes autoBoxSize measure what draw.io draws: <br> is a
  line, other inline tags are invisible.

Graphviz (HTML-like table labels), D2 (grid containers + markdown-in-shape)
and TeX (box+glue) converged on exactly this design: nested boxes for block
structure, proportional glue, a small inline set for text — never full HTML.

Prompts teach the composition with a comparison-card recipe; verified by
rebuilding the CoT poster end to end in the real editor.

lib/diagram-engine/operations.ts

@@ -98,6 +98,18 @@ export const OperationSchema = z.discriminatedUnion("op", [
             .describe(
                 "Flowchart outline: decision=diamond, terminator=start/end, data=input/output, document=report. Omit for a plain rectangle",
             ),
+        grow: z
+            .number()
+            .optional()
+            .describe(
+                "Flex-grow weight: this box takes that share of the parent's leftover space along its stacking axis. Omit for natural size",
+            ),
+        align: z
+            .enum(["start", "center", "end", "stretch"])
+            .optional()
+            .describe(
+                "Cross-axis position in the parent: start/end pin to an edge, stretch fills the axis (a divider or highlight bar spanning its card). Default center",
+            ),
         lane: z
             .number()
             .optional()
@@ -148,6 +160,24 @@ export const OperationSchema = z.discriminatedUnion("op", [
                 "Group stencil name, e.g. 'group_vpc'; omit for a plain frame",
             ),
         gap: z.number().optional(),
+        pad: z
+            .number()
+            .optional()
+            .describe(
+                "Interior padding px (default 24). Small values make tight cards; nest containers for internal structure",
+            ),
+        grow: z
+            .number()
+            .optional()
+            .describe(
+                "Flex-grow weight: this container takes that share of the parent's leftover space. E.g. two columns with grow 2 and 1 split the width 2:1",
+            ),
+        align: z
+            .enum(["start", "center", "end", "stretch"])
+            .optional()
+            .describe(
+                "Cross-axis position in the parent: start/end pin to an edge, stretch fills. Default center",
+            ),
         after: z.string().optional(),
     }),
     z.object({
@@ -414,6 +444,10 @@ export function applyOperations(
                         ...(op.stroke ? { stroke: op.stroke } : {}),
                         ...(op.role ? { role: op.role } : {}),
                         ...(op.group ? { group: op.group } : {}),
+                        ...(op.grow && op.grow > 0 ? { grow: op.grow } : {}),
+                        ...(op.align && op.align !== "center"
+                            ? { align: op.align }
+                            : {}),
                         ...cellOf(op),
                     }
                 else if (op.op === "add_container")
@@ -427,6 +461,11 @@ export function applyOperations(
                         children: [],
                         ...(op.role ? { role: op.role } : {}),
                         ...(op.group ? { group: op.group } : {}),
+                        ...(op.grow && op.grow > 0 ? { grow: op.grow } : {}),
+                        ...(op.align && op.align !== "center"
+                            ? { align: op.align }
+                            : {}),
+                        ...(op.pad != null ? { pad: Math.max(0, op.pad) } : {}),
                     }
                 else if (op.op === "add_grid")
                     node = {