docs: audit all 'what's next' sections against the code; fix stale comments

Verified every deferred item in the nine docs with a what's-next section and
marked what has since landed (reaper, per-process CR3, higher-half kernel,
kernel heap, contiguous frame alloc, RSDP capture, cap-passing, driver restart
with backoff, PS/2 keyboard) while keeping the genuinely open items. Also
corrects interrupts.md's claim that the keyboard skipped the IO-APIC, and
updates scheduler.zig/heap.zig comments that predated the reaper and the big
kernel lock.

docs/interrupts.md

@@ -129,10 +129,11 @@ Both items originally deferred here have landed:
 - **The IO-APIC**: [ioapic.zig](../system/kernel/architecture/x86_64/ioapic.zig)
   routes external device lines onto vectors — discovered via ACPI's MADT, every
   input masked at init, lines unmasked one at a time as user-space drivers bind
-  them (see [device-interrupts.md](device-interrupts.md)). The keyboard turned
-  out not to need it: danos's keyboard is USB HID over xHCI, which interrupts via
-  MSI, not an ISA line. The IO-APIC path is still exercised — e.g. by the HPET's
-  GSI routing.
+  them (see [device-interrupts.md](device-interrupts.md)). The keyboard followed
+  exactly as predicted: the PS/2 bus driver (`system/drivers/ps2-bus/`) claims
+  the 8042 controller and binds its IRQ 1 (and the aux mouse's IRQ 12) through
+  this routing. USB HID keyboards arrive over xHCI instead, which interrupts via
+  MSI, and the HPET's GSI routing exercises the same path.
 - **SSE state**: `isr_common` (and the syscall entry) now `fxsave`/`fxrstor` the
   full SSE/x87 register file around dispatch. This stopped being optional the
   moment kernel code touched XMM — a 16-byte struct copy is a `movdqu` — and its