Replace the Serilog-typed LogOptions.Logger with an injectable Microsoft.Extensions.Logging factory (SetFactory/TrySetFactory), source-generated [LoggerMessage] logging in FileEx/Download, a Serilog-wired Sandbox, and an AwesomeAssertions test suite. Ships as 3.7.

Remove the accidentally-committed coverage report and gitignore coverage output so it cannot recur. Tooling-only; no shipped inputs change.

Promotes the 3.7 release from develop to main.

**Contents**
- #381 - Adopt an injectable `Microsoft.Extensions.Logging` logging
model: `LogOptions.SetFactory(ILoggerFactory)` replaces the removed
Serilog-typed `LogOptions.Logger`; source-generated `[LoggerMessage]`
logging in `FileEx`/`Download`; `Extensions` renamed to
`CompressExtensions`; Serilog-wired `Sandbox`; test suite migrated to
AwesomeAssertions.
- #382 - Gitignore test coverage output and drop an
accidentally-committed report.

**Release impact:** merging publishes the stable **3.7** package to
NuGet (breaking API changes: `LogOptions.Logger` removed, `Extensions`
renamed). No issues to close.

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