Built device interrupts

src/main.zig

@@ -91,6 +91,11 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     con.print("\ndanos: paging enabled\n", .{});
     con.print("  page tables: CR3 = 0x{x:0>16}\n", .{arch.readCr3()});
 
+    // Start the timer and unmask interrupts — the kernel now has a heartbeat.
+    arch.startTimer();
+    arch.enableInterrupts();
+    con.write("\ndanos: timer interrupts enabled\n");
+
     // In a test build (`zig build -Dtest-case=<name>`), run that case and stop.
     // Normal builds fall through to the idle halt.
     if (build_options.test_case) |case| {