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.

build.zig

@@ -788,8 +788,8 @@ pub fn build(b: *std.Build) void {
         "/usr/local/share/qemu/edk2-i386-vars.fd", // macOS Homebrew (Intel)
     });
 
-    // The FHS zig-out (installed above) *is* the boot volume — no separate ESP to
-    // assemble. QEMU presents it to the guest as a FAT drive below.
+    // The guest boots the self-contained FAT image (attached as USB storage below),
+    // not the installed FHS zig-out — see the run step's drive/device flags.
 
     // The firmware needs to write NVRAM, so give it a writable copy of the vars.
     const vars_copy = b.addSystemCommand(&.{ "cp", "-f", ovmf_vars });