Align M3.5 design doc with the nudgeEpoch guard

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/superpowers/specs/2026-05-31-m3.5-semantic-nudge-design.md

@@ -145,14 +145,14 @@ both.
    nudge eligibility:
    - `c.nudge != nil`, runtime is Active, `len(c.recentTitles) >= 2`, and
      `lastNudgedAt` is zero or `now.Sub(lastNudgedAt) >= nudgeDebounce`.
-   - If eligible: stamp `lastNudgedAt = now`, capture `gen := c.driftGen`, the
-     commitment string (same `NextAction — SuccessCondition` form as the drift
-     judge), and a **copy** of `recentTitles`; return the nudge closure.
+   - If eligible: stamp `lastNudgedAt = now`, capture `epoch := c.nudgeEpoch`,
+     the commitment string (same `NextAction — SuccessCondition` form as the
+     drift judge), and a **copy** of `recentTitles`; return the nudge closure.
    - If not eligible: return `nil` (unchanged behavior).
 3. **Nudge closure** (runs in the goroutine):
    - Calls `nudge.Nudge(ctx, commitment, titles)` under a `nudgeTimeout`
      context.
-   - Re-acquires the lock; if `gen != c.driftGen || c.runtimeState !=
+   - Re-acquires the lock; if `epoch != c.nudgeEpoch || c.runtimeState !=
      RuntimeActive` → stale, return.
    - On error → log, leave `nudgeMessage` unchanged (no fabrication), unlock,
      return. (`lastNudgedAt` stays set, so a failed call does not immediately
@@ -209,8 +209,8 @@ avoid stale interrupts after a restart).
   nudge never fabricates a concern, mirroring the drift judge's "never fabricate
   drift" rule.
 - A reasonless concern from the model degrades to silence (see parsing).
-- Stale results (session ended or restarted mid-call) are discarded by the
-  `driftGen` guard.
+- Stale results (the on-task stretch ended via a drift episode, or the session
+  ended/restarted mid-call) are discarded by the `nudgeEpoch` guard.
 
 ## Testing