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.

system/kernel/architecture/x86_64/isr.s

@@ -1,6 +1,6 @@
 # x86_64 low-level entry code: the CPU-exception stubs, plus the GDT/IDT load
 # helpers. Kept in a dedicated assembly file rather than inline asm because these
-# need real labels and cross-symbol jumps/calls (isr_common, exceptionHandler),
+# need real labels and cross-symbol jumps/calls (isr_common, interruptDispatch),
 # and because `lgdt`/`lidt` memory operands aren't expressible in Zig inline asm.
 #
 # Each exception vector normalises the stack to a uniform trap frame — a dummy

system/services/acpi/acpi.zig

@@ -400,8 +400,8 @@ fn isSubscriber(task: u32) bool {
 }
 
 /// Enter S5 (soft off): write SLP_TYP|SLP_EN to the PM1 control register(s).
-/// Mirrors the kernel's power.zig sleepValue. Only reached from a PID-1
-/// shutdown request (M21.3).
+/// The kernel has no S5 path — soft-off is owned here, from the service's own
+/// AML parse. Only reached from a PID-1 shutdown request (M21.3).
 fn enterS5() void {
     if (!s5_valid or pm1a_cnt == 0) {
         _ = runtime.system.write("power: S5 unavailable\n");