chore: land the remaining low-severity items from the whole-project review

Phase 11 of PROJECT_REVIEW_PLAN.md: the themed cleanup pass over every
low-severity finding still open (2.2-2.3, 3.4-3.6, 4.3-4.7, 6.4-6.7,
7.1-7.3, 8.2-8.3, 9.1-9.4, and the under-documented decisions in §10/§11).

Behavioral fixes:
- Reassign duplicate job IDs in a hand-edited jobs.json instead of letting
  two jobs share one runtime, schedule entry, and SeedStats bucket.
- Disambiguate run-log file names that collide within the same second.
- Compute AvgDurationMS as DurationSumMS/TimedRunCount instead of an
  incremental integer mean, so it always matches the seeded-from-logs
  average instead of drifting from truncation error.
- Clean absolute paths in ResolveConfiguredPath so two spellings of the
  same jobs file do not trigger a spurious adoption.
- Report InstallDesktopIcon failures through ErrorOccurred instead of
  discarding them silently.
- Move settingsView's blocking AutostartStatus (PowerShell on Windows) off
  the UI thread.
- Give notify-timing.tsv its own extension so CleanupLogs no longer
  manages it as a run log.
- Replace the settingsView Save handler's second copy of validateConfig's
  rules with a bare parse, letting the Service's own error surface.

Cleanups:
- Delete collectActivity, the dead yaml tags on RunRecord, and the
  logArguments/LogArguments alias.
- Fold the two systemTrayRegistered/mainWindowHidden globals into one
  trayState instance Run owns and threads through Settings and the
  single-instance reveal path.
- Fix stale comments/docs: the frozen window-size restore claim, a
  reference to a renamed recordRun, README's "Pause all" and notification
  wording, the PowerShell quoting note for TESTS.md's coverage command,
  and scripts/test.bat's UTF-8 checkmarks under a non-UTF-8 code page.
- Document the single-instance fallback's consequence and the
  unauthenticated instance-channel port in STANDARDS.md; record the
  config-shim retirement plan in ROADMAP.md.

3.5, 7.3, and 9.4 turned out to already be fixed by earlier phases; no
change needed for those three.

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

src/app/run_test.go

@@ -103,6 +103,15 @@ func TestUpdateStats(t *testing.T) {
 	if rt.AvgDurationMS != 233 {
 		t.Errorf("after run 3: avg=%d, want 233", rt.AvgDurationMS)
 	}
+	// AvgDurationMS must always be exactly DurationSumMS/TimedRunCount — a stored
+	// sum divided once, not an incremental mean that truncates on every step and
+	// compounds error over a long-running job.
+	if rt.DurationSumMS != 700 {
+		t.Errorf("DurationSumMS = %d, want 700", rt.DurationSumMS)
+	}
+	if want := rt.DurationSumMS / int64(rt.TimedRunCount); rt.AvgDurationMS != want {
+		t.Errorf("AvgDurationMS = %d, want DurationSumMS/TimedRunCount = %d", rt.AvgDurationMS, want)
+	}
 }
 
 func TestUpdateStatsSkipsZeroDuration(t *testing.T) {