Assets: OAP reader and mount set on IContentSource #243

Current State

OapReader (OapReader.cs:64-88, 270-313) opens a FileStream and seeks per section; OapMountSet (OapMountSet.cs:35-164) uses File.Exists, Directory.EnumerateFiles for overlays and a FileInfo fingerprint checked on every read (EnsureUnchanged).

Tasks

• OapReader reads from an IContentSource
• OapMountSet mounts sources and detects changes by identity, not per-read stat
• Overlays enumerate through the source
• Keep editor file-watching behavior

Acceptance Criteria

• OAP tests green; desktop export runs
• No File.* in the OAP runtime path
• No load-time regression on example-01

Related

• Android plan task A7 · seniority Mid · mergeable now (.NET 10)
• Depends on: Assets: IContentSource contract for packaged and loose content #242
• Zstd for OAP: Adopt NativeCompressions: Zstd codec for .oap packages and LZ4 for the asset cache #133
• Platform roadmap: Platform roadmap research: Steam Deck, Android, iOS, Web #100