USB2/1.1 (EHCI/UHCI) drivers + real-hardware bring-up hardening

- Add polled EHCI (USB2) and UHCI (USB1.1) host-controller drivers with
  HID boot mouse support, wired into cursor polling and the 'usb' shell cmd
- Add USB Legacy ownership handoff (claim_usb_legacy) plus config_write_byte
- Harden boot: verify the PIT/timer ticks before shell entry, EOI stray
  interrupts in the LAPIC to stop interrupt storms, bound PS/2 controller
  waits, and INTx-disable AHCI (polled driver)
- Add boot stage markers, klog screen echo, and QR lock; scheduler
  mark_shell_running; NO_USB stopgap to keep BIOS USB-Legacy input alive
- alloc_pages_above for DMA buffers that must avoid low conventional memory
- Expand DEBUG_NOTES with EHCI/QEMU bring-up findings

1 files changed, 10 insertions, 0 deletions

diff --git a/rsnix/src/klog.rs b/rsnix/src/klog.rs
index 2e1fc82..300a597 100644
--- a/rsnix/src/klog.rs
+++ b/rsnix/src/klog.rs
@@ -95,6 +95,16 @@ pub fn emit(args: core::fmt::Arguments<'_>) {
     crate::serial::write_raw(line.as_str());
     crate::serial::write_raw("\r\n");
     append(line.as_str(), ms);
+    // Screen echo: mirror the serial line to the VGA console, so the whole
+    // boot log is visible on real hardware without a serial console.  The
+    // last line on screen when the machine freezes is exactly where boot
+    // stopped (far more precise than the stage marker).  Skipped while a
+    // fault dump is rendering its own band (row 0 = boot marker, terminal
+    // lives below it).
+    if crate::vt::ready() && !crate::interrupts::in_dump() && !crate::gop::qr_locked() {
+        crate::vt::write_str(line.as_str());
+        crate::vt::write_str("\n");
+    }
     crate::sched::unlock();
 }