interrupts: IDT, PIC remap, PIT timer, PS/2 keyboard IRQ

- IDT with 256 entries, loaded via lidt (CS read from current context)
- PIC 8259A remapped to vectors 32-47, timer+keyboard IRQs unmasked
- PIT channel 0 at ~100 Hz (mode 2, rate generator)
- PS/2 keyboard IRQ (IRQ1 → vector 33): reads port 0x60, pushes byte
  to lock-free SPSC ring buffer
- Timer IRQ (IRQ0 → vector 32): sends EOI, increments tick counter
- ps2::poll_key() now reads from ring buffer with cli/sti+hlt for
  race-free sleep (instead of busy-polling the data port)
- build.rs: use idiomatic Result<_, Box<dyn Error>> return type
- tools/qemu.sh: fix SC2207 shellcheck warning

1 files changed, 2 insertions, 2 deletions

diff --git a/build.rs b/build.rs
index 44e6344..35447e9 100644
--- a/build.rs
+++ b/build.rs
@@ -26,8 +26,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         return Err("not a PSF2 font".into());
     }
 
-    let hdr_size =     u32::from_le_bytes(raw[8..12].try_into()?) as usize;
-    let num_glyphs =   u32::from_le_bytes(raw[16..20].try_into()?) as usize;
+    let hdr_size = u32::from_le_bytes(raw[8..12].try_into()?) as usize;
+    let num_glyphs = u32::from_le_bytes(raw[16..20].try_into()?) as usize;
     let bytes_per_glyph = u32::from_le_bytes(raw[20..24].try_into()?) as usize;
 
     if num_glyphs != 256 {