Terminal-app design (ADR 0005) + disk-write controller

- ADR 0005: persistent home terminal (full-screen, chrome-less,
  bottom-pinned), F1 terminal windows, per-instance vt/tty/shell,
  queue-based input for all terminals, and the disk controller
- Glossary: add the Terminal term; update Task/App for the new model
- Add a re-entrant disk lock (DiskGuard over the preemption-depth
  counter) and wrap all fs/fat mutating operations per-call

1 files changed, 11 insertions, 3 deletions

diff --git a/CONTEXT.md b/CONTEXT.md
index 9fccbe4..c27f059 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -113,9 +113,16 @@ The action that destroys a window and ends its app.
 
 A task that owns exactly one window and receives input events through its
 message queue.  A GUI program is an app; there is no separate "process"
-concept — the app is its task plus its window.  The console is app 0
+concept — the app is its task plus its window.  The home terminal is app 0
 (window 0).
 
+## Terminal
+
+A program that presents a command line in a window: a text terminal
+emulator plus a shell command loop, rendered into the window's pixel
+buffer.  The home terminal is the persistent full-screen chrome-less
+desktop; additional terminals are ordinary windows (F1).
+
 ## Focus
 
 The single window (if any) that receives keyboard and mouse input.  Focus
@@ -142,8 +149,9 @@ 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
+fallback.  A task may sleep, yield, and log to serial/klog; disk access
+is serialized by a shared disk controller (a re-entrant lock), and each
+terminal is its own program instance.  A panicking or
 stack-overflowing task dies gracefully (logged, marked done, slot
 reaped) without taking the kernel down.