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/loopd/tests/broker_port_bad.rs

@@ -9,8 +9,10 @@ use std::os::unix::net::UnixStream;
 use std::thread;
 use std::time::Duration;
 
+use bxcore::{
+    Envelope, ErrorCode, Message, PROTOCOL_VERSION, ToolResponse, WireError, write_frame,
+};
 use fake::{assert_unavailable, broker, call, frame, in_ms, request, result, send};
-use proto::{Envelope, ErrorCode, Message, PROTOCOL_VERSION, ToolResponse, WireError, write_frame};
 
 #[test]
 fn frames_that_break_the_protocol_are_unavailable() {