feat(core,engine): "Cinephile" achievement for completing a replay

Adds a 19th achievement: "Cinephile — Watch a saved replay all the
way through." Unlocks the first time ReplayPlaybackState transitions
Playing → Completed (i.e. the move list runs out without the player
pressing Stop). Discoverability nudge for the replay feature itself.

The achievement uses the existing event-driven unlock pattern
(condition closure returns false; an unlock system fires
AchievementUnlockedEvent on the right state transition) rather than
the standard condition-evaluation path, mirroring how other
non-stat-driven achievements work.

The unlock system distinguishes natural completion from Stop-button
abort by watching for the specific Playing → Completed transition;
Stop transitions Playing → Inactive directly without going through
Completed, so it doesn't fire the achievement. Already-unlocked
state is checked via AchievementsResource so the achievement can't
double-fire on subsequent replays.

README's "18 Achievements" → "19 Achievements". ARCHITECTURE.md §11
gains a Cinephile entry alongside the existing 18.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ARCHITECTURE.md

@@ -716,11 +716,14 @@ pub struct AchievementDef {
 | `speed_and_skill` | ??? | Win < 90s without undo | Yes | Card back #4 |
 | `comeback` | ??? | Win after 3+ stock recycles | Yes | Background #4 |
 | `zen_winner` | ??? | Win in Zen Mode | Yes | Badge |
+| `cinephile` | Cinephile | Watch a saved replay all the way through | No | — |
 
 ### Evaluation Timing
 
 Achievement conditions are evaluated by `AchievementPlugin` on every `GameWonEvent` and `StateChangedEvent`. The plugin calls `solitaire_core::check_achievements()` which returns a `Vec<AchievementDef>` of newly unlocked achievements. The plugin then fires `AchievementUnlockedEvent` for each, which the toast and persistence systems handle independently.
 
+A small number of achievements are *event-driven* rather than condition-driven: their `AchievementDef::condition` always returns `false` and their unlock is written from a dedicated observer system instead. `cinephile` is the canonical example — it unlocks when `ReplayPlaybackState` transitions from `Playing` to `Completed` (a saved replay watched to its natural end). The Stop button transitions `Playing → Inactive` directly without entering `Completed`, so manual aborts do not unlock the achievement.
+
 ---
 
 ## 12. Progression System