Serve saved Smooth Streaming fragment files offline

[harshvasudeva]

Summary

• Add offline playback support for locally saved Smooth Streaming fragment files.
• Keep existing complete-track .ism behavior intact.
• If no preloaded indexed track exists for a requested fragment, look for a saved file at the matching request-path layout: Episode/QualityLevels(<bitrate>)/Fragments(<track>=<start time>).
• Also serve a local manifest file for fragment-based downloads.

Why

The current Xbox/Akamai CDN still serves client manifests and fragment URLs, but the older full server-manifest/media-file flow used by QuantumFetcher is no longer available for at least current Steam installs. The companion QuantumFetcher PR can download fragments directly; this PR lets QuantumStreamer serve those files back to the game.

Testing

• Built Release x64 locally with vcpkg manifest dependencies (poco via vcpkg).
• Installed the resulting loc_x64_f.dll into a Steam Quantum Break install.
• Verified a patched QuantumFetcher test download produced local fragment layout for J1A: 2,253 files, ~139 MB.

Companion PR needed in QuantumFetcher to produce the fragment file layout.