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/audit_startup.rs

@@ -1,13 +1,13 @@
 //! What `Writer::open` does with the log it finds: refuse a broken chain, recover a torn tail,
 //! accept a break when told to. Do not edit. The fixture logs are in
-//! `crates/proto/tests/fixtures/audit/`; each test works on a copy.
+//! `crates/bxcore/tests/fixtures/audit/`; each test works on a copy.
 
 #[path = "support/audit_dir.rs"]
 mod audit_dir;
 
 use audit_dir::{D1, D2, TempDir, denied, lines, snapshot};
 use brokerd::audit::{AuditError, Writer, verify_dir};
-use proto::{AuditEvent, AuditRecord, Location, Timestamp};
+use bxcore::{AuditEvent, AuditRecord, Location, Timestamp};
 
 type Case = (
     &'static str,