test the trampoline is inert (zeroed + NX) when dormant

Adds paging.isExecutable and arch.trampolinePage; the smp case now asserts the frame is zeroed and non-executable after bring-up. Teeth-checked against a no-op disarm.

src/kernel/tests.zig

@@ -483,6 +483,20 @@ fn smpTest() void {
     }
     log("DANOS-SMP: workers ran on {d} distinct core(s)\n", .{cores_seen});
     check("tasks ran on multiple cores in parallel", cores_seen >= 2);
+
+    // Bring-up is done, so the trampoline frame must be inert: zeroed (no stale code)
+    // and non-executable (W^X restored). It's armed only while a core is climbing.
+    const tramp = arch.trampolinePage();
+    check("trampoline frame reserved", tramp != 0);
+    if (tramp != 0) {
+        const bytes: [*]const u8 = @ptrFromInt(tramp);
+        var zeroed = true;
+        for (0..4096) |b| {
+            if (bytes[b] != 0) zeroed = false;
+        }
+        check("trampoline page zeroed when dormant", zeroed);
+        check("trampoline page non-executable when dormant", !arch.pageExecutable(tramp));
+    }
     result();
 }