docs: API reference (docfx)

[benaadams]

Problem

No API documentation exists. Consumers need reference docs for all public types.

Sub-tasks

• Add XML doc comments to all public types, methods, and properties
• Configure docfx (or similar) to generate HTML API reference
• CI step to build docs and catch missing XML comments
• Host on GitHub Pages

Dependencies

Depends on: #108, #116, #128 (all public APIs should exist before documenting)
Unlocks: #138 (NuGet Source Link + XML docs)

Phase

Phase 4 (Polish & Release Readiness)