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>

README.md

@@ -22,7 +22,7 @@ optional self-hosted sync so your stats follow you across machines.
   move within picker rows, Enter activates; works across every modal and
   the HUD action bar
 - **Progression** — XP, levels, unlockable card backs and backgrounds
-- **18 Achievements** — including secret ones
+- **19 Achievements** — including secret ones
 - **Daily Challenge** — server-seeded so every player worldwide gets the
   same deal
 - **Leaderboard** — opt-in, powered by your own self-hosted server