Add extensible Achievement API

[HayashiUme]

Overview

Add an extensible achievement API to MiraAPI. The API provides only data logic
— no UI, no networking. The Example project demonstrates how to build title
display, RPC sync, and chat commands on top of it.

API (MiraAPI/Achievements/)

• IAchievement — interface for defining custom achievements
• BaseAchievement — abstract base with built-in progress tracking
• AchievementManager — registry, progress, titles, encrypted file persistence
• AchievementTier — rarity enum (Common → Legendary)
• AchievementUnlockedEvent — MiraEvent fired on first unlock

Persistence

• Stored alongside existing presets:
  Application.persistentDataPath/mira_achievement/{ModGuid}/Achievement.dat
• Same layout as mira_presets/ — one folder per mod GUID
• XOR encryption with a key derived from the mod GUID

Example (MiraAPI.Example/Achievements/)

• MadKillerAchievement / FirstVictoryAchievement — sample achievements
• /achievement <id> — equip an unlocked title in chat
• /achievement unlock <id> — force-unlock for testing
• AchievementTitleComponent — shows [Title] above player name
• TitleSyncRpc — syncs equipped title across network

Scope

This is the minimal foundation. Extension ideas that consumers can build on top:

• Achievement UI — browse all achievements, view progress bars, equip titles
• Cloud stats — upload progress to a server for leaderboards or cross-device sync
• Seasonal achievements — time-limited achievements that auto-expire

Notes

Extensive XML doc comments are included on all public members to fully
demonstrate the API surface. Some phrasing may be slightly off — I'm not a
native English speaker and relied on translation tools for parts of the docs.

[HayashiUme]

Due to some overlap with existing APIs, it has been converted to Draft upon request. If you remain interested, you can restart and merge it into the main branch