feat(diagram-engine): corners, borderless fills, shadows and strikethrough

Four more Tailwind classes, all four verified against draw.io's own source in
public/drawio rather than against a prose reference — which is how three earlier
exclusions turned out to be wrong:

  rounded-*      mxShape.js:1172-1189 — absoluteArcSize=1 switches arcSize to
                 absolute pixels and halves it, so the same class is the same
                 corner on every box. Previously excluded as 'percentage only'
  shadow-sm..xl  mxShape.js:505-535 — getShadowStyle reads five independent
                 params, not one flag, so Tailwind's offset+blur rungs map one
                 to one. Previously excluded as 'six sizes collapse to one'
  line-through   mxConstants.js:2054 FONT_STRIKETHROUGH: 8, read at both
                 mxText.js:723 and :1040. The bitmask has four bits, not three
  border-none    the only one of the four that adds something previously
                 inexpressible: a fill with no outline

Also fixes an edge style growing 76 characters per re-layout, without bound. The
router recomputes ports on every pass, and appending them to a style recovered
from the canvas — which already carried the previous pass's eight port keys —
grew the string forever. draw.io resolves duplicates last-wins so the arrow
always looked right; a byte-identity check is what caught it.

Two traps found while wiring the readback, both the same shape: a value the
THEME emits being recorded as one the model asked for. strokeColor=none from a
filled or ghost role, and rounded=0 from the fallback style. Either one would
outlive a set_role, since that clears style but keeps text.

Deliberately not included, with reasons in tw.ts: per-side borders and per-corner
radius (both would take the shape slot, and what a node IS matters more than
which of its edges show), per-side padding (draw.io's keys pad the label, not the
room left for children), text-shadow (a bare flag with no offset or blur),
opacity (Tailwind's is any integer, not a scale), tracking/uppercase/leading
(absent from draw.io — zero grep hits, not merely coarse).

615 tests, 90 new. Browser-verified: four shadow rungs visibly differ, radius is
real pixels, terminator + rounded-lg becomes a small-cornered rounded rect while
an untouched terminator stays a stadium.

tests/unit/diagram-engine-graph.test.ts

@@ -1,9 +1,9 @@
 import { describe, expect, it } from "vitest"
 import {
-    drawGraph,
     type GraphEdge,
     type GraphNode,
     graphToOperations,
+    type Operation,
     restructureDiagram,
 } from "@/lib/diagram-engine"
 import {
@@ -20,6 +20,31 @@ const e = (source: string, target: string, label?: string): GraphEdge => ({
     ...(label ? { label } : {}),
 })
 
+/**
+ * Draw a whole-page graph the way the model does: one `add_graph` with no parent.
+ *
+ * There used to be a `drawGraph` entry point that did only this. It was removed once
+ * `add_graph` covered the same ground with a `parent` argument — one code path instead of
+ * two overlapping ones — and these tests kept their assertions by going through this.
+ */
+function drawGraph(
+    nodes: GraphNode[],
+    edges: GraphEdge[],
+    opts: { title?: string; flow?: "col" | "row" } = {},
+) {
+    const ops: Operation[] = [
+        {
+            op: "add_graph",
+            id: "g",
+            nodes,
+            edges,
+            ...(opts.flow ? { dir: opts.flow } : {}),
+        } as Operation,
+    ]
+    if (opts.title) ops.unshift({ op: "set_title", title: opts.title })
+    return restructureDiagram("", ops)
+}
+
 describe("graphToOperations: layering", () => {
     it("puts a chain in one node per layer", () => {
         const { layers } = graphToOperations(