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-roundtrip.test.ts

@@ -348,6 +348,35 @@ describe("edges survive", () => {
         })
         expect(renderDiagram(t).xml).not.toContain('as="points"')
     })
+
+    it("an edge's style does not grow across repeated re-layouts", () => {
+        // The router recomputes the connection points every pass, and an edge recovered from
+        // the canvas already carries the previous pass's set. Appending them added 76
+        // characters per round-trip forever. draw.io resolves duplicate keys last-wins, so
+        // the arrow always LOOKED right — only measuring the string catches it.
+        let t = tree([group("f", "row", [icon("a"), icon("b")], "F")], {
+            links: [{ source: "a", target: "b" }],
+        })
+        const styleOfEdge = (xml: string) =>
+            /<mxCell id="ed1"[^>]*style="([^"]*)"/.exec(xml)?.[1] ?? ""
+
+        const lengths: number[] = []
+        const portCounts: number[] = []
+        let xml = ""
+        for (let pass = 0; pass < 4; pass++) {
+            xml = renderDiagram(t).xml
+            const s = styleOfEdge(xml)
+            lengths.push(s.length)
+            portCounts.push((s.match(/exitX=/g) ?? []).length)
+            t = parseDiagram(xml).tree
+        }
+
+        // Same length every pass, and the port keys stated once rather than accumulating.
+        expect(new Set(lengths).size).toBe(1)
+        expect(portCounts).toEqual([1, 1, 1, 1])
+        // Which is what lets the XML itself reach a fixed point.
+        expect(renderDiagram(parseDiagram(xml).tree).xml).toBe(xml)
+    })
 })
 
 describe("foreign cells survive", () => {