fix: execute canonical browser download targets

Replace the boolean browser-managed target validator with
resolveBrowserManagedTarget, which returns the parsed canonical absolute URL,
and hand that exact value to the host. Previously the raw href was passed on,
so a relative target was re-resolved against document.baseURI and a hostile
<base> could send the navigation to an origin the policy never approved.

STO-08 stays UNVERIFIED: the capability spec does not exercise the system
picker, so the receiver-binding hypothesis is neither reproduced nor refuted
and no source change was made for it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/browser-file-download.test.ts

@@ -248,12 +248,49 @@ describe("browser download delivery", () => {
       ok: true,
       value: { kind: "BROWSER_HANDOFF", transferId: "transfer:1" },
     });
+    // STO-02. Parse once, then execute exactly what was validated.
     expect(handoff).toHaveBeenCalledWith(
-      "/downloads/artifact-1",
+      "https://app.example/downloads/artifact-1",
       "invoice_exe.pdf",
     );
   });
 
+  it("executes the canonical target instead of a document-base-relative href", async () => {
+    const handoff = vi.fn();
+    const adapter = createDownloadDeliveryAdapter({
+      ...HARD_LIMITS,
+      policies,
+      host: { handoff },
+      // A relative href the host would otherwise resolve against a hostile
+      // document <base href="https://evil.example/">.
+      browserManagedCapabilities: capabilityResolver(() => "downloads/a"),
+      baseOrigin: "https://app.example",
+      createTransferId: () => "transfer:1",
+      userActivation: { isActive: true },
+    });
+
+    expect(
+      await adapter.deliver(
+        deliveryInput(
+          {
+            kind: "BROWSER_MANAGED_RESOURCE",
+            resourceId: "artifact-1",
+            capabilityReceipt,
+          },
+          "BROWSER_MANAGED",
+        ),
+      ),
+    ).toMatchObject({ ok: true });
+    expect(handoff).toHaveBeenCalledWith(
+      "https://app.example/downloads/a",
+      "invoice_exe.pdf",
+    );
+    for (const [href] of handoff.mock.calls) {
+      expect(String(href).startsWith("https://app.example/")).toBe(true);
+      expect(String(href)).not.toContain("evil.example");
+    }
+  });
+
   it("rejects cross-origin or query-bearing browser-managed targets", async () => {
     const handoff = vi.fn();
     const adapter = createDownloadDeliveryAdapter({