feat: retro mode — arcade combo round with gamepad overlays

Every fight now includes one Retro Mode round where bots submit
gamepad combo inputs (↑↓←→ A B). 24 moves across 4 tiers: basic
(always shown), standard (partially revealed), super (must discover),
and ultra (KONAMI CODE for 50 dmg). Discovery bonus gives 1.5x damage.

Includes pixel-art gamepad overlays (P1/P2) with animated button
presses, retro-specific narrations, and mock bot combo responses
scaled by ELO.

Also adds loops/plan.md with 11-phase production hardening roadmap.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/FightViewer.vue

@@ -360,6 +360,8 @@ async function _doReplay() {
         isCritical,
         botAScore: round.botAScore || 0,
         botBScore: round.botBScore || 0,
+        botAResponse: round.challengeType === 'retro_mode' ? (round.botAResponse || undefined) : undefined,
+        botBResponse: round.challengeType === 'retro_mode' ? (round.botBResponse || undefined) : undefined,
       })
     } catch (err) {
       console.error(`[FightViewer] playRound ${round.roundNumber} error:`, err)