docs: full docs-vs-code audit — fix every stale claim across 40 docs

Every doc verified claim-by-claim against the code by parallel audit agents,
then fixed and adversarially re-verified. Two waves of staleness corrected:
the originally audited findings (higher-half boot handoff, kernel VFS
takeover, fault isolation + claim release + driver restart, AML/S5 moving to
ring 3, threading's shipped design, USB+FAT landing) and a second pass of
adjacent claims the verifiers caught (smp.md 'not built yet' intro,
system-requirements' PS/2-only and no-storage claims, halting.md's red-panic
and no-IDT text, testing.md's serial mirroring, router-era vfs-protocol
wording, capsule-first boot loading).

threading.md now documents the shared-fate gap explicitly: the design says a
process dies whole, the kernel today kills only the offending thread.

Also fixes three stale code comments (isr.s exceptionHandler, acpi.zig
sleepValue, build.zig boot-volume) — comments only, no behavior change.

docs/vision.md

@@ -90,11 +90,12 @@ own page tables — plus a [test harness](testing.md).
 - **Isolation track** — **user mode + address-space isolation**. *Done: a
   higher-half kernel with a physmap (the low half is user space), per-process
   address spaces with CR3 switched on context switch, the `swapgs` discipline,
-  `syscall`/`sysret`, a user-ELF loader, and `/system/services/init` running as a real
-  preemptive ring-3 process (PID 1). Remaining polish: an address-space/stack
-  reaper for exited tasks, SMAP + fault-recovering copy-in/out, the real IPC
-  syscalls (IPC_Call/IPC_ReplyWait — they arrive with the second user server),
-  and TLB shootdown once a process has more than one thread.*
+  `syscall`/`sysret`, a user-ELF loader, an address-space/stack reaper for exited
+  tasks, and `/system/services/init` running as a real preemptive ring-3 process
+  (PID 1). Remaining polish: SMAP + fault-recovering copy-in/out, and TLB shootdown
+  once a process has more than one thread. (The real IPC syscalls —
+  `ipc_call`/`ipc_reply_wait` — have since been built and are the backbone every
+  driver and service speaks; see [ipc.md](ipc.md).)*
 - **Resilience track** — fault → kill → notify, a supervisor/reincarnation server,
   resource cleanup on death, then a restartable driver as proof. Needs isolation.
   See [resilience.md](resilience.md).