W06.B: make-install smoke + cross-watch gap discovery

[espadonne]

Summary

W06.B of the W campaign. Does not ship a fix — ships the
smoke (tests/smoke/make-install-undo-fbsd.sh) that documents
the cross-watch capture gap and discovers an unexpected
infrastructure issue underneath it.

The unexpected finding

W06 was scoped as "wire the existing LD_PRELOAD shim's
notifications into the journal." The smoke discovered that
the shim isn't even firing its interposers on FreeBSD — zero
shim notifications reach the daemon's listener for any
LD_PRELOAD'd command, including direct LD_PRELOAD=shim touch /tmp/x; rm /tmp/x. ldd shows all deps resolve. Target
binaries (install, make, touch, rm) are all dynamically
linked.

So W06's scope is bigger than initially thought:

1. First: validate + repair shim load + interpose on FreeBSD
   (pre-S24.D.2 work that was incompletely validated)
2. Then: wire shim notifications → journal events
3. Then: pre-image capture via shim, make-install smoke
   green, W08 case re-run, CI wiring

That's a campaign analogous to W07.A — see
.docs/sprints/W/W06.B-bsd.md §Discoveries for the full
diagnostic.

Test plan

• All 8 W smokes on shit-fbsd:
  • rm-undo: PASS
  • vim-edit-undo: PASS
  • cp-recursive-undo: PASS
  • sed-inline-undo: PASS
  • cargo-build-undo: PASS
  • dd-large-file-undo: PASS (Outcome A streaming)
  • mv-across-dirs-undo: PASS (documenting W06 territory)
  • make-install-undo: PASS (documenting W06 territory)
• freebsd-smoke (B04) CI green