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/brokerd/tests/serve_accept_break.rs

@@ -58,7 +58,7 @@ fn a_break_is_accepted_and_the_next_plain_start_serves() {
     std::fs::create_dir_all(&audit).unwrap();
     std::fs::create_dir_all(dir.path().join("grants")).unwrap();
     let fixture =
-        Path::new(env!("CARGO_MANIFEST_DIR")).join("../proto/tests/fixtures/audit/changed-byte");
+        Path::new(env!("CARGO_MANIFEST_DIR")).join("../bxcore/tests/fixtures/audit/changed-byte");
     // The damage is in the first file; an ordinary start checks only the latest, so copy that one
     // alone, as `audit_startup.rs` does.
     std::fs::copy(