feat(diagram-engine): design tokens + role/group composition, engine-wide theming

Paper-summary posters previously required hand-written XML: every engine
box rendered identically (white, 11px), so anything whose meaning lives
in visual hierarchy came out flat. This makes presentation a first-class,
generalised part of the declaration - not a poster feature.

Structure/presentation separation, the same split HTML and CSS settled on:

- ROLE says what a node IS: banner, heading, body, callout, good, bad,
  metric, muted. Maps to a type scale and an emphasis (filled / tinted /
  outlined / ghost), never to a colour.
- GROUP says which semantic zone a node belongs to. Each distinct group
  name gets one hue ramp (tint / base / dark), assigned in document
  order. Promoted from a draw_graph-only field to BoxNode and GroupNode,
  round-tripped via dai_group.
- themedStyle(role, hue, kind) composes the two by rule - there is no
  per-combination table to extend, so a new diagram kind gets full
  theming by tagging nodes. The model never sees a hex value.

A heading container plus a group yields the tinted section panel with a
dark title; a grouped body box takes its zone's tint; verdict roles stay
green/red regardless of zone; the banner is the page's one dark field.

Also fixed, found while building the acceptance poster:

- autoBoxSize only counted explicit newlines, so a long single-line label
  wrapped to six lines in draw.io but got a one-line-tall box, and the
  text overflowed the cell.
- Marker stamping appended without replacing, so every render of a
  recovered style grew it by one duplicate dai_* token per key -
  unnoticed because draw.io resolves duplicates last-wins. dai_* keys
  are now replaced in place; mxGraph keys still append, because
  last-wins is load-bearing for container=1 normalisation.
- Banner/heading/metric roles stretch across their container's cross
  axis, the way a masthead spans its page.
- Prompt: a poster's banner IS its title (no set_title alongside), and
  sections get their colour by naming groups.

537 unit tests, 250-flowchart corpus still zero crossing arrows, 5 e2e
tests in a real browser. Verified visually: the Transformer-paper poster
renders with a navy masthead, three hue-coded section panels, metric,
verdict and callout boxes - all engine-computed geometry.

lib/diagram-engine/parse.ts

@@ -36,6 +36,7 @@ import {
     readList,
     readMarker,
 } from "./markers"
+import { isRole, type Role } from "./theme"
 import type {
     BoxNode,
     BoxShape,
@@ -347,6 +348,18 @@ function looksLikeText(style: string): boolean {
 }
 
 /** The flowchart outline a box is drawn with, read back from its style. */
+/** The information role stamped on a cell, if any. */
+function roleOf(style: string): Role | undefined {
+    const v = readMarker(style, MARKER.role)
+    return isRole(v) ? v : undefined
+}
+
+/** The semantic zone stamped on a cell, if any. */
+function zoneOf(style: string): string | undefined {
+    const v = readMarker(style, MARKER.group)
+    return v ? decodeURIComponent(v) : undefined
+}
+
 function boxShape(style: string): BoxShape | undefined {
     const shape = styleValue(style, "shape")
     if (shape === "parallelogram") return "data"
@@ -1055,6 +1068,8 @@ export function parseDiagram(xml: string, pageIndex = 0): ParseResult {
                 fill: styleValue(c.style, "fillColor"),
                 stroke: styleValue(c.style, "strokeColor"),
                 shape: boxShape(c.style),
+                role: roleOf(c.style),
+                group: zoneOf(c.style),
                 // A lifeline's style is chrome the renderer rebuilds, so keeping it verbatim
                 // would re-emit a lifeline that no longer matches the new message count.
                 style: lifeline ? undefined : c.style,
@@ -1204,6 +1219,8 @@ export function parseDiagram(xml: string, pageIndex = 0): ParseResult {
             kind: "group",
             ...common,
             dir: dir === "col" ? "col" : "row",
+            role: roleOf(c.style),
+            group: zoneOf(c.style),
         }
         return node
     }