logging: root every layer's XLOG category (standalone dep maps + picoquic bridge + smoke test)

[gmarzot]

Makes every per-layer --logging selector work, and adds a regression test. Three parts, one theme — each XLOG layer is rooted at its natural category name.

Problem

Every bundled dependency compiles with an absolute __FILE__, so folly derives categories like home.runner.work.moxygen...MoQSession. Result: --logging=moxygen=DBG4, quic=WARN, fizz=DBG1, quic.picoquic=DBG3, etc. match nothing — only the root level (--logging=DBG4) works. The per-layer operator UX documented in moqx docs/logging.md and the PicoQuicXLogSink header is effectively dead.

Fix — three complementary pieces

1. C++ dependencies (standalone/CMakeLists.txt). We don't own their CMakeLists, so root the categories from the consumer side: before each dep's add_subdirectory, add -fmacro-prefix-map to strip its FetchContent source root from __FILE__. folly then derives folly.*, fizz.*, wangle.*, quic.mvfst.*, proxygen.*, moxygen.*.

-fmacro-prefix-map (not -ffile-prefix-map) is deliberate: rewrites only __FILE__/macros (debug-info paths intact), stacks across deps without macro-redefinition conflicts, and shortens __FILE__ before folly's own (ineffective-when-embedded) FOLLY_XLOG_STRIP_PREFIXES runs — neutralizing that latent bug too. mvfst is mapped under quic/mvfst/ (the documented category) so it doesn't collide with picoquic at a bare quic.* root. The moxygen map roots moxygen.* immediately, independent of the upstream moxygen fix (facebookexperimental#207).

2. picoquic bridge (PicoQuicXLogSink.cpp). The sink used bare XLOG(), so its events landed under openmoq.transport.pico.* instead of the advertised quic.picoquic.*. Add XLOG_SET_CATEGORY_NAME("quic.picoquic") — the right model for a bridge: it names its category explicitly rather than deriving it from __FILE__, so it needs no prefix-map.

3. Regression test (standalone/test/xlog_category_smoke.sh, CTest xlog_category_roots). Drives a short date-server/text-client session and asserts, per layer, that its selector enables that layer's DBG-level lines and excludes the others. Matches only V-prefixed (DBG) lines so INFO/WARN/ERR can'"'"'t mask a broken category; layers still on the glog backend are skipped, not failed. Validated against a pre-fix build: it correctly FAILs moxygen scoping while sanity-detecting the same lines at the root level.

These categories bake into the CI-built artifacts, so downstream consumers (e.g. the moqx relay) inherit the clean roots automatically.

Relationship to #207

facebookexperimental#207 fixes only moxygen.* (moxygen'"'"'s own CMakeLists). This PR covers the other five layers plus picoquic plus the test; the one moxygen map here is redundant-but-harmless once #207 lands and syncs.

Test plan

• CI standalone build (deps on XLOG via MOXYGEN_LOGGING_FOLLY_XLOG=ON) runs ctest -R xlog_category_roots.
• Manual: --logging=moxygen.MoQSession=DBG4 scopes to moxygen; --logging=quic.mvfst=DBG1 to transport; --logging=quic.picoquic=DBG3 to the picoquic bridge.

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This change is 

[afrind]

@afrind reviewed 3 files and all commit messages, and made 3 comments.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on gmarzot and michalhosna).

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standalone/CMakeLists.txt line 476 at r1 (raw file):

    # stacks across deps without macro-redefinition conflicts, and shortens
    # __FILE__ *before* folly's own (CMAKE_SOURCE_DIR-scoped, thus ineffective
    # here) FOLLY_XLOG_STRIP_PREFIXES runs — neutralizing that latent bug too.

Same here. I recently learned that claude needs to "talk to itself" in the comments in order to write good code, and needs either a pre-tool hook or a second pass to clean this up to something humans actually want.

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standalone/CMakeLists.txt line 586 at r1 (raw file):

moxygen_install_headers(moxygen ${CMAKE_CURRENT_SOURCE_DIR}/../moxygen)

# Regression guard for the per-layer XLOG category rooting above: drives short

Does this belong here in the standalone/CMakeLists.txt top level? Maybe it's better in a sub-file (under test?)

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moxygen/openmoq/transport/pico/PicoQuicXLogSink.cpp line 26 at r1 (raw file):

// overriding the file-path-derived default (openmoq.transport.pico.*). This is
// the semantic root operators target (`--logging=quic.picoquic=DBG3`), and it
// keeps picoquic independent of the build-side prefix-maps: unlike the C++ deps,

Consider whether we can shorten this comment a bit?