proto becomes bxcore, the one shared crate (M5b task 00)

Done the way the owner asked and the spike mapped: the crate and the
manifests renamed, then what the build reported fixed. Layers: 138
"proto unresolved" errors across 169 files; cargo fmt reordering
imports; six fixture paths built as strings in brokerd's and bxctl's
tests, which the compiler cannot see, and comments naming
crates/proto/; the crate-dependency check and its self-test. One more
the spike did not reach: a `\nproto =` key inside a printf format in
the self-test, which a word-boundary replacement skips because `n` and
`p` are both word characters.

make gate: gate: ok, 944 tests; no new entry in /tmp.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/bxctl/tests/verify_skipped.rs

@@ -9,7 +9,7 @@ fn home(tag: &str) -> temp::TempDir {
     let dir = temp::TempDir::new(&format!("bx-verify-skip-{tag}"));
     std::fs::create_dir_all(dir.join("audit")).unwrap();
     let fixture =
-        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../proto/tests/fixtures/audit/good");
+        PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("../bxcore/tests/fixtures/audit/good");
     for entry in std::fs::read_dir(fixture).unwrap() {
         let entry = entry.unwrap();
         std::fs::copy(entry.path(), dir.join("audit").join(entry.file_name())).unwrap();