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-embed-graph.test.ts

@@ -95,7 +95,7 @@ describe("add_graph", () => {
         expect(right.w).toBeGreaterThan(right.h)
     })
 
-    it("reports unknown edge endpoints as an error", () => {
+    it("draws the rest of the graph and warns about an unknown edge endpoint", () => {
         const r = restructureDiagram("", [
             {
                 op: "add_graph",
@@ -104,6 +104,10 @@ describe("add_graph", () => {
                 edges: [{ source: "a", target: "ghost" }],
             },
         ])
-        expect(r.errors.join(" ")).toContain("ghost")
+        // A warning, not an error: node "a" is perfectly drawable, and rejecting the whole
+        // call would cost a turn to arrive back at the same diagram.
+        expect(r.errors).toEqual([])
+        expect(r.xml).toBeTruthy()
+        expect(r.warnings.join(" ")).toContain("ghost")
     })
 })