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/brokerd/src/broker.rs

@@ -9,7 +9,7 @@ use std::sync::Mutex;
 use std::sync::mpsc::{Receiver, RecvTimeoutError};
 use std::time::Duration;
 
-use proto::{
+use bxcore::{
     DenyReason, Envelope, ErrorCode, FrameError, Message, PROTOCOL_VERSION, PendingApproval,
     Timestamp, ToolResponse, WireError, read_frame, write_frame,
 };
@@ -37,7 +37,7 @@ pub struct Broker {
     table: Table,
     runtime: Box<dyn Runtime>,
     log: Log,
-    printed: Mutex<Option<Vec<proto::GrantProblem>>>,
+    printed: Mutex<Option<Vec<bxcore::GrantProblem>>>,
 }
 
 impl Broker {