Init folly XLOG (async by default) and root categories at moqx.*

[gmarzot]

Summary

Two small consumer-side changes that complete the moqx half of the logging normalization:

1. src/main.cpp — declare a compile-time baseline logging config so async writes are on by default:

   FOLLY_INIT_LOGGING_CONFIG(".=INFO; default:async=true,sync_level=WARN");

   Keeps the relay off the log-I/O path; `--logging=...` and the `FOLLY_LOGGING` env var still override at startup.

2. CMakeLists.txt — root XLOG categories at `moqx.` instead of `src.`:

   add_compile_options(-fmacro-prefix-map=${CMAKE_CURRENT_SOURCE_DIR}/src=moqx)
   add_compile_definitions(
     "FOLLY_XLOG_STRIP_PREFIXES=\"${CMAKE_SOURCE_DIR}:${CMAKE_BINARY_DIR}\""
   )

   moqx sources live under `src/`, but the user-facing logging API expects `moqx` as the top-level category. The macro-prefix-map rewrites `FILE` so xlog auto-derives `moqx.MoqxRelay` etc. The strip-prefixes define covers paths the prefix-map doesn't touch (generated headers under the build dir).

Pairs with upstream moxygen-side work landing the same FOLLY_XLOG_STRIP_PREFIXES pattern there and converting moxygen sources to XLOG.

Test plan

• Build moqx; verify `--logging moqx=DBG4` selects moqx-rooted categories.
• Verify async writes (`default:async=true,sync_level=WARN`) — INFO/DBG don't block, WARN+ flushed synchronously.
• `--logging=...` runtime override still works.

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