Merge #547: record the finally non-goal on drainAll's completion line

Comment only. No behaviour change.

Verified by me before merging:
- `mvn -B clean install` in a scratch worktree: exit 0, Tests run: 1716, Failures: 0, Errors: 0,
  Skipped: 0. Same count as main, as expected for a javadoc-only change.
- #459's javadoc reference gate: exit 0, 0 reference errors.
- Gitea CI run 1801 on 5eb4267: success.

The constraint came from the fleet01 lead. Their point: the value of the `drain complete` line is
that it is MISSING when a drain does not finish. A `finally` block would print it after a drain
that threw, with partial counts, and destroy both halves at once. The code already avoids this;
what was missing was the sentence that stops a reviewer putting it back.

fleetd/src/main/java/dev/ltms/fleet/session/SessionManager.java

@@ -1074,6 +1074,29 @@ public final class SessionManager implements TurnListener {
      * members were live) and must still produce the line. Both {@link #drainSnapshot} passes (the
      * main snapshot and the straggler sweep) are folded into the one line: a caller reading two
      * lines could not tell a two-pass drain from two separate drains.
+     *
+     * <p><strong>Non-goal: this line must never move into a {@code finally} block, and this method
+     * must never grow one around it.</strong> "Every time" above means every time the drain
+     * <em>finishes</em>, not every time this method exits. The absence of the line is the signal
+     * that the drain died, so a {@code finally} would destroy the signal and print confident
+     * partial counts in the same edit — the line would appear after a drain that threw, carrying
+     * whatever {@code tally} it had reached. Both halves of the value are lost at once. The line
+     * has to be the last statement of the successful path and reachable only from it.
+     *
+     * <p>This is written down because it is the obvious review comment ("shouldn't we always log
+     * the drain result?"), it sounds like thoroughness, and the paragraph above reads as an
+     * invitation to it. Raised by the fleet01 lead on 2026-09-12, from their 2026-09-10 incident:
+     * we only know that drain died on that host because it <em>threw</em>, and a
+     * {@code NoClassDefFoundError} reached the JVM's uncaught handler. A drain that hung on one
+     * session, or returned early on a condition rather than an exception, would leave no stack
+     * trace, no {@code ERROR} token and no priority — only a missing line. That makes the loud
+     * variant the one we have seen and the quiet variants the ones this line exists to catch.
+     *
+     * <p>Related: {@code released} and {@code abandoned} are counted incrementally inside {@link
+     * #drainSnapshot}'s loop and folded with {@link DrainTally#plus}, rather than derived from a
+     * collection read at the end, for the same reason. If a partial report is ever wanted it must
+     * be a different line with a different verb. One line must not serve both, or a reader cannot
+     * tell a finished drain from an interrupted one by its wording.
      */
     void drainAll(long timeoutNanos) {
         long deadline = System.nanoTime() + timeoutNanos;