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/tests/chat.rs

@@ -2,11 +2,11 @@
 
 #[path = "support/temp.rs"]
 mod temp;
-use bxctl::chat::{ChatError, Printer, new_session_id, run_turn};
-use proto::{
+use bxcore::{
     DataClass, Envelope, ErrorCode, Message, PROTOCOL_VERSION, SessionId, Turn, TurnDone,
     TurnEvent, Usage, WireError, read_frame, write_frame,
 };
+use bxctl::chat::{ChatError, Printer, new_session_id, run_turn};
 use std::os::unix::net::UnixListener;
 use std::path::PathBuf;
 use std::process::Command;