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, 97 insertions, 0 deletions

diff --git a/docs/adr/0003-preemptive-kernel-tasks.md b/docs/adr/0003-preemptive-kernel-tasks.md
new file mode 100644
index 0000000..ac38027
--- /dev/null
+++ b/docs/adr/0003-preemptive-kernel-tasks.md
@@ -0,0 +1,97 @@
+# ADR 0003 — Preemptive Kernel Tasks
+
+## Status
+
+Accepted.
+
+## Context
+
+The kernel is single-threaded and polling-based: a shell loop drives
+everything, drivers poll (AHCI has no interrupts), and the only
+preemption source is a 100 Hz PIT tick whose ISR currently just counts
+and acknowledges.  The interrupt path already saves a complete register
+frame (`isr_common` pushes all GPRs + the iretq frame onto the
+interrupted stack), so switching tasks is "return a different stack
+pointer from the ISR dispatch" — a small asm change, not new machinery.
+
+The destination is a windowed GUI running multiple applications at
+once, which requires concurrent execution and, eventually, per-task
+input/output channels.  The immediate goal is the foundation: a
+preemptive multitasking core that the GUI will be built on.
+
+Two constraints shape the design.  First, all kernel driver state is
+single-user by construction: one shared AHCI DMA buffer plus a global
+"active disk" selector, a per-volume cached FAT with a global active
+volume, the VFS node table, and the shell's tty/vt terminal.  Two tasks
+doing disk or filesystem work concurrently would corrupt each other.
+Second, there is no address-space separation and no user/kernel
+boundary — the kernel is one identity-mapped space (TempleOS-style), so
+"tasks" (not processes) sharing every kernel facility is the honest
+model.
+
+## Decision
+
+Preemptive, round-robin **kernel tasks** in the one shared address
+space, preempted on the 100 Hz PIT tick:
+
+- **Scheduler** — a fixed 16-slot static task table (the timer ISR
+  never allocates), round-robin across Ready tasks, one class, 10 ms
+  quantum.  States: `ready`, `sleeping` (wake-at timestamp from
+  `uptime_ms`), `blocked`, `done`.  The shell is task 0; a kernel idle
+  task (`hlt` loop) is the always-ready fallback.  Reaping is lazy:
+  Done tasks' stacks are freed and slots reused at the next `spawn` (and
+  `jobs` sweeps).
+- **Context switch** — the timer ISR saves the running task's frame on
+  its own stack, stores the frame pointer in the task's slot, picks the
+  next Ready task, and returns its frame pointer; the asm moves `rsp`
+  there and iretq's.  Yield and sleep use a software interrupt (vector
+  0x40) through the same machinery.
+- **Stacks** — each task gets 8 KiB from the page allocator plus one
+  **guard page** mapped below it (the paging code already maps/unmaps).
+  A new page-fault handler (vector 14) reads CR2; a fault inside a
+  task's guard page is a "stack overflow" task panic, any other fault
+  halts the kernel with the address.
+- **Spawn surface** — `sched::spawn(entry, name)` is general (any plain
+  `fn`), but the shell only offers safe builtins: `bg <name>`,
+  `jobs`, `sleep <ms>`, `kill <id>`, `demo`.  First-slice tasks are
+  headless: they may sleep, yield, and log to serial/klog — nothing
+  else.  Invariant: **only the shell task touches the disk,
+  filesystem, and terminal**; the scheduler does not enforce this (a
+  general `spawn` cannot), it is a documented boundary.
+- **Line atomicity** — a scheduler lock (preempt-disable counter, "no
+  switching while held") is held by `klog::append_fmt` and
+  `serial::write_str` so lines from different tasks never interleave
+  mid-line.  The timer ISR skips switching while the lock is held.
+- **Graceful task panic** — the panic handler becomes task-aware: if
+  the scheduler is live and the panic is in a task (id ≥ 1), it logs
+  `sched: task N "name" panicked: <msg>`, marks the task Done, clears
+  the scheduler lock, and switches away via vector 0x40 — the shell
+  keeps running and `jobs` shows the corpse.  Panics in ISR context,
+  before the scheduler exists, or in the shell task itself still halt.
+- **Shell surface** — `bg <name>` spawns a builtin (`spinner`,
+  `ticker`, `counter`, plus `boom` which panics on purpose), `jobs`
+  lists id/name/state, `sleep <ms>` blocks the shell task (background
+  tasks keep running — the visible proof of preemption), `kill <id>`
+  marks a task Done, `demo` spawns a showcase set at different sleep
+  rates.
+
+Explicitly **not** in this ADR: backgrounding arbitrary shell commands
+(`cmd &`), per-task I/O channels, task isolation, priorities, and the
+GUI itself.  The scheduler's blocking/wake primitives are designed so
+the GUI's per-task windows and input queues can be built on them later.
+
+## Consequences
+
+- Concurrent disk/fs/tty/vt access remains unsupported: any second task
+  doing I/O needs per-task DMA buffers and real mutexes, which is
+  future work (the GUI-era step).
+- Task stacks overflow into a clean, labeled task death instead of
+  silent corruption; the glossary's "no guard pages" claim now reads
+  "except task stacks".
+- A buggy background task cannot take the kernel down (panic or stack
+  overflow), which the GUI will depend on; a buggy shell still can.
+- The serial log becomes interleaved across tasks — expected, and the
+  scheduler lock keeps it line-clean.
+- klog timestamps make task activity (`[ssss.mmm]`) traceable alongside
+  boot messages; rescan events, panic messages, and task logs all share
+  the ring.