[C1] Install-route detection test fixture

[radical]

Track C validation. Depends on B0 (#33).

Add a test fixture under tests/Aspire.Acquisition.Tests/ that simulates each InstallRoute (script / pr-script / winget / brew / dotnet-tool / unknown) by setting up a fake binary path layout + sentinel file, and asserts:

1. InstallRouteDetector returns the expected route + evidence.
2. aspire update --self --dry-run prints the right plan per route.
3. aspire uninstall --self --dry-run prints the right plan per route.

Uses the mocked-fs + mocked-gh-CLI infrastructure from microsoft#15995.

Scenarios covered

S1.1–S1.6 (route detection), S2.1–S2.6 (--self dispatch), S6.1–S6.5 (uninstall dispatch).

---

Part of: #29