ldls: L4 was free after all, and the plan closes

L4 was listed as "free -- H2 built all of this" and never actually tested,
which is a claim rather than a result. Tested now, and it holds:

    ldlsd running     ping 2/2; holds index 7 and DMA slots 3 and 4
    killed            interface DOWN, device released, bus mastering
                      cleared, DMA reclaimed -- back to vgpud's 3 blocks
    ping              fails honestly: sendto: -5
    a second ldlsd    ping 3/3, and the arena hands back the same addresses

pciuser_release_all() and netuser_release_all() sit beside each other in
exit() for the same reason, and that they apply unchanged to a foreign driver
is the whole point of having built them against our own.

With that, every milestone on ldls.md is done, so it leaves docs/plan/ for
docs/architecture/ -- the rule docs/plan/README.md states and the one that
keeps the two directories meaning different things. The new document describes
what is, not what was planned: the five rules, the four pieces, what it costs
(polling, no stack protector, locks that are no-ops), that it can be killed,
and a "what it does not do" that leads with one driver, one kernel version,
tried once.

Sixteen source files pointed at docs/plan/ldls.md and would have been dangling
the moment it moved. All repointed, and the milestone references -- which the
architecture doc deliberately does not have -- rewritten to name sections
instead, so a reader lands somewhere real.

The register row closes with it. hybrid.md freed a slot, ldls.md took it, and
it is now free again.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

ldls/ko.h

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0
 #pragma once
 
-// Loading a relocatable object. docs/plan/ldls.md milestone L0.
+// Loading a relocatable object. docs/architecture/ldls.md, "The loader".
 //
 // A Linux `.ko` is an ordinary ELF64 relocatable object -- the same thing
 // `gcc -c` produces -- and loading one is four steps with no mystery in any of