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/support/client.rs

@@ -11,7 +11,7 @@ use std::sync::Arc;
 use std::time::Duration;
 
 use brokerd::broker::{self, Broker};
-use proto::{Envelope, Message, PROTOCOL_VERSION, ToolRequest};
+use bxcore::{Envelope, Message, PROTOCOL_VERSION, ToolRequest};
 
 use crate::rig::Rig;
 use crate::runtime::{Recording, Shared};
@@ -49,7 +49,7 @@ pub fn open(
         r#final: true,
         msg,
     };
-    proto::write_frame(&mut client, &env).unwrap();
+    bxcore::write_frame(&mut client, &env).unwrap();
     client
 }
 
@@ -60,7 +60,7 @@ pub fn next(stream: &mut UnixStream) -> Envelope {
         let peer_closed = cfg!(target_vendor = "apple") && e.raw_os_error() == Some(22);
         assert!(peer_closed, "set_read_timeout: {e}");
     }
-    proto::read_frame(stream).unwrap()
+    bxcore::read_frame(stream).unwrap()
 }
 
 /// Sends a tool request to `broker::handle` and reads every frame up to the final one.