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/display-plan.md

@@ -7,7 +7,8 @@ Read [display.md](display.md) first for the *why*; this is the *what* and the *o
 ## Locked decisions (do not relitigate)
 
 - **Handoff = device node + write-combining `mmio_map`.** The kernel seeds a synthetic
-  `display0` node from `BootInformation.framebuffer`; the service claims + WC-maps it.
+  display node (found by class, not name) from `BootInformation.framebuffer`; the
+  service claims + WC-maps it.
   (Not a bespoke `framebuffer_map` syscall — the device route inherits ownership,
   release-on-death, and re-claim-on-restart.)
 - **v1 = the full compositor pipeline on the dumb framebuffer.** One `display` service