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/approvals_judge.rs

@@ -3,9 +3,9 @@
 
 use std::sync::mpsc::{self, Receiver};
 
+use bxcore::ApprovalReply;
 use gatewayd::approvals::{Judged, NOT_APPROVER, Posted, Registry, USAGE};
 use gatewayd::sessions::ApprovalCommand::{self, Approve, Deny, Malformed};
-use proto::ApprovalReply;
 
 const KYLE: &str = "k0000000000000000000000000";
 const HERMES: &str = "h0000000000000000000000000";