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/support/gateway.rs

@@ -8,10 +8,10 @@ use std::sync::{Arc, Mutex, mpsc};
 use std::thread::JoinHandle;
 use std::time::Duration;
 
+use bxcore::Turn;
 use gatewayd::config::Config;
 use gatewayd::secrets::Secret;
 use gatewayd::serve::{Stop, Tuning, run};
-use proto::Turn;
 
 use crate::fake_loop::{done, serve_loop};
 use crate::fake_mm::{FakeMm, KYLE, WsPeer};