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/tests/ledger_label.rs

@@ -15,7 +15,7 @@ mod tmp;
 use brokerd::ledger::Call;
 use brokerd::policy::{Label, SessionState};
 use build::now;
-use proto::{AuditEvent, CallId, DataClass, SessionId, ToolResponse};
+use bxcore::{AuditEvent, CallId, DataClass, SessionId, ToolResponse};
 use rig::Rig;
 
 fn label(class: DataClass, untrusted: bool) -> Label {