feat: auto cash-out targets, 1% house edge, terminal aesthetic

Auto cash-out closes a position at exactly the chosen target rather than
the next tick's multiplier, and fires whenever the target is at or below
the crash point. This is the feature that makes the game playable over a
network, where manual timing is at the mercy of latency.

House edge drops from 2% to 1% across crash and scratch. Scratch prize
tables retuned so the published 99% RTP is exact.

Adds docs/API.md: the client uses no private endpoints, so anyone can
write a bot against the same API.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

pkg/sim/crash.go

@@ -6,8 +6,13 @@ import (
 	"github.com/drjones/quantum-arcade/pkg/fixed"
 )
 
-// HouseEdgeBP is the house edge in basis points (200 = 2.00%).
-const HouseEdgeBP int64 = 200
+// HouseEdgeBP is the house edge in basis points (100 = 1.00%).
+//
+// One percent is deliberately generous — better than almost anything
+// commercial. This is a game among friends, not a revenue stream, and a
+// thinner edge means the pot lasts the whole night instead of draining
+// toward the house.
+const HouseEdgeBP int64 = 100
 
 // TickHz is the simulation rate. Rounds advance in whole ticks only.
 const TickHz = 60