Add loopback pprof listener

[jerenkrantz]

Summary

• add an opt-in loopback-only pprof listener configured by PLAID_GOCACHE_PPROF_ADDR or the serve --pprof-addr flag
• serve the standard Go runtime profiling endpoints on a dedicated mux and listener
• document the listener and add focused configuration, lifecycle, route-isolation, and flag tests

Testing

• Not run (intentionally deferred).