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.

system/kernel/heap.zig

@@ -9,8 +9,9 @@
 //! of free blocks, split on allocation and coalesced with neighbours on free. It
 //! is exposed as a std.mem.Allocator, so the kernel can use std containers.
 //!
-//! Not yet concurrency-safe: it assumes a single caller and no allocation from
-//! interrupt handlers (ours don't). A lock comes with threads/SMP.
+//! No lock of its own: every kernel entry takes the big kernel lock (sync.zig),
+//! which serializes all heap access. A private lock only becomes necessary if
+//! the big lock is ever split.
 
 const std = @import("std");
 const abi = @import("abi");