Fix compile-time resilience: use registry file as fallback for unresponsive instances

During long Unity compilations, the main thread is blocked and the HTTP
bridge can't respond to pings. Previously this caused validateSelectedInstance()
to falsely clear the selection, thinking the project was gone.

Now uses the instance registry file (persists across compiles) as a secondary
signal: if a port is unresponsive but the registry still lists our project
there, we keep the selection and trust that Unity is just compiling.

Selection is only cleared when we have positive evidence the project is gone:
not responding AND not listed in the registry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>