Add comprehensive feature documentation with platform requirement analysis

[Copilot]

Documents all Spark features with analysis of which capabilities require C#, Windows OS, or local agent execution vs. could be platform-agnostic.

Added FEATURES.md

33+ feature categories covering:

• Real-time API polling (30/15Hz), replay recording (.echoreplay, .milk formats), event detection
• NVIDIA Highlights, OBS/Medal.tv integration, browser overlays
• Camera control, Quest discovery, Discord Rich Presence
• TTS/speech recognition, global hotkeys, HID device support
• Process management, WebSocket/REST APIs, local database

Platform Requirement Analysis

Requires Windows:

• WPF UI, Registry access, Win32 APIs (hotkeys, focus), WebView2
• NVIDIA Highlights SDK, NAudio (CoreAudio), system tray

Requires Local Agent:

• Game API polling (127.0.0.1:6721), process control, file system access
• Local hardware: NVIDIA GPU, HID devices, audio capture
• Quest network scanning (ARP table), camera write to game instance

Platform-Agnostic:

• Event detection algorithms, overlay frontend (Svelte), replay format parsing
• WebSocket/REST servers, database operations, Discord integration

C# Specific:

• WPF/Windows Forms, ASP.NET Core (though alternatives exist in any language)
• Benefits from async/await, LINQ, but not strictly required

Summary Matrix

Includes quick-reference table showing which features could be cloud-hosted vs. require Windows desktop vs. need local execution.

Key insight: Core functionality requires Windows because EchoVR is Windows-only, but underlying algorithms and data formats are portable. Alternative implementations could use Electron, Qt, Avalonia, or native frameworks.

Original prompt

Generate a comprehensive feature list of this app.

then provide insight into which features can only be achieved by an application written in C#, what requires an agent on the local windows machine, etc.

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