SRE #84 phases 1 and 2. Five files, ~30 load-bearing lines, and the point is
not tidiness: on a box where the account is not `pln`, SuperDirt booted
completely cleanly with every one of the three sample roots silently absent,
and the LCXL painter units exec-failed while rig-doctor reported the LCXL
section green. Two silent failures, each standing behind something that looked
like a pass.

**Phase 1 — the three sample roots and the two same-repo loads.**
start_and_midi.scd and sample_watch.scd held /home/pln as literal strings.
The fix is not a blind sed, because a raw tilde expands nowhere by itself.
Three traps, all verified on sclang 3.14.1 before writing a line:

  - `"~/x".standardizePath` keeps a trailing glob intact, so the loadSoundFiles
    calls can stay one-liners. Good.
  - `File.exists("~/x")` returns FALSE. A naive tilde substitution would have
    left the preload and sample-watcher guards syntactically perfect and
    permanently false — a dead preload with no error, which is worse than the
    hardcoded path it replaced.
  - `thisProcess.nowExecutingPath` is nil inside s.waitForBoot's closure. The
    two same-repo loads live in there, so the repo root is captured at TOP
    level into ~pvRepoRoot and used later. Reading it in place would have
    produced exactly the same dead-guard failure as the tilde.

Deriving the repo root from the file's own location beats a home-relative
guess: the rig now runs from any clone path, not merely any username.

**Phase 2 — the two outlier units.** lcxl3-driver.service and
lcxl-leds-watch.service baked WorkingDirectory and an absolute ExecStart,
where every sibling unit (midiviz, perf-tray, parvagues-bridge,
midi-autoconnect, tidal-ardour-autoroute) already used %h. Now they do too,
verified with systemd-analyze --user verify.

**Phase 3, unplanned — the fix broke the check that guards the fix.**
rig-doctor's check_scd_sample_roots reads the roots OUT of start_and_midi.scd
rather than hardcoding them, which is the right design and is why it caught
this at all. But it then called Path.is_dir() on the extracted literal, and
Python does not expand a tilde either — the same trap as File.exists, one
language over. So the portability fix turned three PASSes into three false
FAILs on the box where all three roots exist. That is how a check gets
switched off. It now expands before testing, and reports the resolved path so
the reader can see what was actually stat'ed.

Validation: both .scd files compile (File.readAllString(f).compile, which
parses without executing); both units pass systemd-analyze verify; rig-doctor
goes from 3 fail / 36 pass to 0 fail / 39 pass with the same verdict logic;
901 passed, 2 skipped across the suite.

The lesson for the log is the third phase, not the first two. A checker that
derives its expectations from the source it checks is strictly better than one
with hardcoded copies — and it still shares the source's bugs, one runtime
removed.