docs: ADR 0003 (preemptive kernel tasks) + task glossary terms

- ADR 0003: preemptive round-robin kernel tasks on the 100 Hz tick,
  shared address space, guard pages + page-fault handler, graceful task
  panics, safe-builtin spawn surface, only-shell-touches-I/O invariant
- CONTEXT.md: new 'Task' glossary entry; 'Paging strategy' now says
  guard pages exist on task stacks

1 files changed, 16 insertions, 1 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index aa2392e..65475ce 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -32,7 +32,9 @@ structs are hand-defined (not from `uefi-rs` crate) to stay lightweight.
 
 Long mode mandates paging (`CR0.PG = 1`), so we accept that tax but keep it
 simple.  Identity-mapped 4-level tables, no higher-half, no demand paging,
-no guard pages.  `paging.rs` provides identity map/unmap/query helpers.  At
+no guard pages except task stacks — each task's stack gets one unmapped
+guard page below it, and a page-fault handler turns an overflow into a
+task panic.  `paging.rs` provides identity map/unmap/query helpers.  At
 boot the kernel runs on UEFI's tables; `adopt_tables()` rebuilds that map
 into the kernel's own tables and switches CR3 — needed because UEFI maps its
 page-table pages read-only, so the firmware tree can't be modified in place.
@@ -80,6 +82,19 @@ linear frame buffer is system memory that we own once boot services are gone.
 Scancode set 1, US QWERTY layout.  Hardcoded scancode→ASCII table.
 Other layouts can be added later by swapping the translation table.
 
+## Task
+
+A unit of concurrent execution in the kernel: its own kernel stack
+(with a guard page below it), scheduler bookkeeping, and no
+address-space separation — all tasks share the one identity-mapped
+space and the kernel's facilities (TempleOS-style, not a process).
+Tasks are preemptively round-robin-scheduled on the 100 Hz PIT tick;
+the shell is task 0 and an idle `hlt` task is the always-ready
+fallback.  A task may sleep, yield, and log to serial/klog; only the
+shell touches the disk, filesystem, and terminal.  A panicking or
+stack-overflowing task dies gracefully (logged, marked done, slot
+reaped) without taking the kernel down.
+
 ## Development loop
 
 Standalone project — no integration with the RNG initramfs build system.