rsnix: preemptive kernel tasks (ADR 0003) — sched, IST fault stacks, 176-byte frames

- sched: 16-slot static table, shell=task 0, idle=task 1, spawn with
  guard page + 8 KiB stack, RR on the 100 Hz tick, int 0x60 yield,
  sleep_ms, lazy reap, jobs/bg/sleep/kill shell commands
- gdt: own GDT mirroring UEFI selectors + TSS with IST1/IST2
  (guard-page overflows die gracefully instead of triple-faulting)
- interrupts: isr_dispatch returns next frame in rax; IntFrame grows
  rsp/ss fields — QEMU's 64-bit iretq always pops all five slots from
  a 176-byte frame, so synthetic task frames must carry rsp=top+ss
  (a 160-byte frame made the first iretq pop rsp/ss from the unmapped
  page beyond the frame)
- panic handler: task panics abort the task, shell keeps running
- main: sched::init moved after page allocator + adopted tables
  (the idle's stack+guard need both; it was silently not spawning)

1 files changed, 2 insertions, 0 deletions

diff --git a/rsnix/src/lib.rs b/rsnix/src/lib.rs
index a2d41da..08ac6c2 100644
--- a/rsnix/src/lib.rs
+++ b/rsnix/src/lib.rs
@@ -6,6 +6,7 @@ pub mod ahci;
 pub mod fat;
 pub mod font;
 pub mod fs;
+pub mod gdt;
 pub mod gop;
 pub mod interrupts;
 pub mod klog;
@@ -15,6 +16,7 @@ pub mod pci;
 pub mod power;
 pub mod ps2;
 pub mod rtc;
+pub mod sched;
 pub mod serial;
 pub mod strbuf;
 pub mod tty;