Nitwit 1.3.1 — Elytra Deployment Latency Fix

Changes
- Integrated a modern Fabric-side port of the deployment behavior from the supplied ElytraFix 1.12.2 reference.
- After vanilla sends START_FALL_FLYING, the client immediately sets its local fall-flying metadata flag (only when a player exists and is not touching water) instead of waiting for a server metadata round-trip to update the local state.
- The mixin matches intermediary names for Minecraft 1.21.1's ClientConnection, ClientCommandC2SPacket, MinecraftClient, ClientPlayerEntity, and Entity flag setter.
- Preserved Nitwit 1.3.0 adaptive performance classes and configuration.

Important limits
- This fixes local Elytra deployment responsiveness; it does not predict or fabricate rocket thrust, override server movement corrections, change a remote server's TPS, or remove network round-trip latency. Rocket momentum and position remain server-authoritative.
- The reference mod's 1.12.2 landing-physics mixin was not blindly copied because Minecraft's modern movement implementation differs.
- JAR/metadata and bytecode structure were checked in this environment. A live Minecraft/Fabric multiplayer test was not available, so test with the exact 1.21.1 setup and keep the prior JAR as a rollback.
