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/gatewayd/tests/deliver.rs

@@ -10,11 +10,11 @@ mod tmp;
 use std::sync::Mutex;
 use std::time::Duration;
 
+use bxcore::{DataClass, ErrorCode, SessionId, Timestamp, TurnEvent};
 use fake_loop::{Reply, done, error, event, serve_loop};
 use gatewayd::deliver::{EMPTY_ANSWER, LOOP_DOWN, MAX_POST, Notice, Poster, deliver, split_answer};
 use gatewayd::mm::MmError;
 use gatewayd::sessions::{Batch, Thread};
-use proto::{DataClass, ErrorCode, SessionId, Timestamp, TurnEvent};
 use tmp::TempDir;
 
 const DM: &str = "d0000000000000000000000000";
@@ -217,11 +217,11 @@ fn a_loop_that_is_not_there_or_goes_away() {
         ],
         vec![Reply::Bytes(b"\x00\x00\x00\x05{bad}".to_vec())],
         vec![
-            Reply::Frame(proto::Envelope {
+            Reply::Frame(bxcore::Envelope {
                 v: 1,
                 id: 2,
                 r#final: true,
-                msg: proto::Message::Ok(proto::Empty {}),
+                msg: bxcore::Message::Ok(bxcore::Empty {}),
             }),
             done("late"),
         ],