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/src/chat.rs

@@ -4,7 +4,7 @@ use std::io::{BufRead, Write};
 use std::os::unix::net::UnixStream;
 use std::path::Path;
 
-use proto::{
+use bxcore::{
     DenyReason, Envelope, ErrorCode, Message, PROTOCOL_VERSION, SessionId, Timestamp, Turn,
     TurnDone, TurnEvent, WireError, read_frame, write_frame,
 };
@@ -15,7 +15,7 @@ use crate::escape::{escape_json_text, escape_model_text};
 #[derive(Debug)]
 pub enum ChatError {
     Connect(std::io::Error),
-    Frame(proto::FrameError),
+    Frame(bxcore::FrameError),
     Refused(WireError),
     Protocol(String),
 }