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-theme.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest"
-import { drawGraph, restructureDiagram } from "@/lib/diagram-engine"
+import { restructureDiagram } from "@/lib/diagram-engine"
 import {
     absoluteRects,
     escapesParent,
@@ -117,18 +117,22 @@ describe("roles", () => {
         expect(third.xml).toBe(again.xml)
     })
 
-    it("draw_graph nodes accept roles too", () => {
-        const r = drawGraph(
-            [
-                { id: "t", label: "Pipeline", role: "heading" },
-                { id: "a", label: "Build" },
-                { id: "warn", label: "Flaky stage", role: "bad" },
-            ],
-            [
-                { source: "t", target: "a" },
-                { source: "a", target: "warn" },
-            ],
-        )
+    it("add_graph nodes accept roles too", () => {
+        const r = restructureDiagram("", [
+            {
+                op: "add_graph",
+                id: "g",
+                nodes: [
+                    { id: "t", label: "Pipeline", role: "heading" },
+                    { id: "a", label: "Build" },
+                    { id: "warn", label: "Flaky stage", role: "bad" },
+                ],
+                edges: [
+                    { source: "t", target: "a" },
+                    { source: "a", target: "warn" },
+                ],
+            },
+        ])
         expect(r.errors).toEqual([])
         expect(last(styleOf(r.xml as string, "warn"), "fillColor")).toBe(
             "#F8CECC",