FABRIC-2.md: ACL-RWT-style 3x3 Latin square re-run on Stadium substrate (item 5.1/F.3)

Found and fixed a real bug before any campaign work could start: EXEC-DOE's
own CSV output was almost entirely lost to console interleaving with the
routine per-tick heartbeat export -- same bug class as Section L's PLOT
case. Fix: HB-OFF immediately before EXEC-DOE, HB-ON after DOE: complete.
Confirmed HB-ON-first (the reverse order) does NOT fix it -- tested
directly, row loss recurred identically.

Also found: L8-DOE/WL-HI/WL-LO (the mechanism bare_metal/README.md
describes as auto-run) don't exist anywhere in capsules/, and Makefile.
starkernel's DOE_SEED variable is declared but never referenced -- both
vestigial, matching Section K's earlier staleness finding.

Built QEMU-serial-socket injection tooling (socat) to drive EXEC-DOE
interactively after boot, since it requires live REPL input, not just
observation. Two real defects found and fixed in that tooling itself: a
log-discovery race (self-excluding the very log it needed to find,
causing two separate stuck-injector incidents, one overnight) and an
unredirected background launch that deadlocked socat on a full stdout
pipe. Both fixed by having the orchestrator pass exact log/socket paths
directly and always launching through the harness's tracked-background
mechanism.

First full campaign attempt ran all 9 cells as three ISA-blocked loops,
reusing one build per architecture -- caught mid-run: this confounds ISA
with time/session-order, invalidating the Latin square design. Discarded
(logs kept as audit artifacts, not treated as valid data) and re-run
clean: all 9 (arch, seed) cells in fully randomized order, fresh clean
rebuild before every single cell, one continuous sitting. Result:
4,320/4,320 rows captured, zero VM errors anywhere.

This validates the campaign mechanism runs cleanly and reproducibly under
the post-4.6 Stadium substrate -- satisfies item 5.1's own concern that a
green POST suite isn't evidence determinism holds post-migration. It does
NOT produce an ACL-RWT overhead number: ACL.4th is not self-activated in
this repo's default init.4th, so these 9 cells ran with ACL inactive.
Reproducing the original +0.0054%-+0.0088% measurement needs a paired
ACL-enabled/disabled run using this now-validated mechanism -- scoped,
not attempted here.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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