test/hil, docs: move the containment history into the design doc

The modules were 21% comment, much of it review-cycle argument rather than
guidance -- _kill_kids stated 'descendant by construction, no argv check needed'
twice, eight lines apart. Deleting such comments outright makes maintenance
worse: the next reader simplifies the thing the comment was defending. So the
history moves to the 2026-07-30 fleet-wedge design doc, which gains a trim
addendum recording what was removed, what was deliberately kept, and the rule
that decided each -- the CI ceiling bounds how long a run burns, and does nothing
about state that outlives it.

One comment was not merely long but WRONG: the report wipe carried 'The unlink is
DEFERRED to inside the pool try/except below', which is the opposite of what the
code does -- it sits before Manager() with its own comment explaining why. That
is the failure mode this pass is about, so it is deleted rather than reworded.

Kept everywhere: citations that refute a plausible wrong reading. That
usb_lock_device_interruptible is why the readers are killable, that usblp_mutex
is driver-global, that rawmidi honours O_NONBLOCK where usblp does not.

Two follow-ups are retired with them: pr3803-hil-blindness-reporting.md (there
is no blindness to report any more) and pr3803-usbtest-recovery-reserve.md (the
reserve is derived now). Kept: pr3803-flasher-recover.md, which PR #3832
implements, plus pr3803-pci-rebind-stranding.md and pr3803-hil-iar-rerun-spec.md,
both independent of this work.