Add historical binary log replay coverage - #14782

Fixes #6587

Context

MSBuild does not currently have focused regression coverage proving that the current BinaryLogReplayEventSource can replay historical binary-log file formats 1 through 14. Without checked-in compatibility fixtures, a reader change could break older .binlog files without being detected by the test suite.

Changes Made

• Added a small canonical MSBuild project used to produce compatibility fixtures.
• Added compact binary-log fixtures for historical file-format versions 1 through 14.
• Added CanReplayHistoricalBinaryLogFormats, which replays each fixture and verifies:
  • the reported file-format version;
  • that events were produced;
  • one project and two targets were replayed; and
  • the recorded build completed successfully.

All new test assertions use Shouldly, as recommended by the contribution guide.

Testing

• Built and ran an isolated validator using the repository-pinned .NET SDK's MSBuild reader (18.10.0-1.26360.111). All 14 fixtures replayed successfully with the expected format version, one project, two targets, and successful completion.
• Built Canonical.proj successfully with zero warnings and zero errors.
• Verified every fixture has a gzip header and a stable SHA-256 digest.
• Verified git diff --check passes.

The full Microsoft.Build.Engine.UnitTests project was not restored locally because the isolated environment does not contain its full dependency graph. No bulk package restore was performed; CI should provide the complete project-level validation.

Notes

• This is a test-only change and does not modify product behavior or the current binary-log format.
• Every fixture records a build of the checked-in Canonical.proj and uses the historical serializer format identified by its filename.
• For the oldest formats whose original toolchains no longer run directly on the current machine, the corresponding historical serializer source was hosted in a compatible generator. No profiler events are present in the fixtures.