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/gateway_sock.rs

@@ -12,12 +12,12 @@ use std::sync::mpsc::{self, Receiver};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
-use gatewayd::gateway::{Ending, Timing, ToLoop, listen, serve};
-use proto::{
+use bxcore::{
     ApprovalOutcome, ApprovalReply, ApprovalRequest, CallId, DataClass, Envelope, ErrorCode,
     GatewayOutcome, Message, PROTOCOL_VERSION, PendingApproval, SessionId, Timestamp, read_frame,
     write_frame,
 };
+use gatewayd::gateway::{Ending, Timing, ToLoop, listen, serve};
 use tmp::TempDir;
 
 const KYLE: &str = "k0000000000000000000000000";