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/scheduling.md

@@ -6,7 +6,7 @@ ready task always runs, and tasks at the same priority take turns. That model is
 chosen for [real-time](vision.md) — it's predictable (you can reason about which
 task runs when) and its decisions are O(1), unlike a fair-share scheduler.
 
-The scheduler proper (`system/kernel/sched.zig`) is generic; the context switch and new-task
+The scheduler proper (`system/kernel/scheduler.zig`) is generic; the context switch and new-task
 stack setup are architecture-specific (`system/kernel/architecture/x86_64/`, see [architecture](architecture.md)).
 
 ## Tasks