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/support/rig.rs

@@ -12,7 +12,7 @@ use brokerd::audit::Writer;
 use brokerd::config::{Approvals, Config, Paths, Sockets};
 use brokerd::ledger::Ledger;
 use brokerd::state::StateStore;
-use proto::{AuditEvent, AuditRecord, CallId, SessionId, ToolRequest};
+use bxcore::{AuditEvent, AuditRecord, CallId, SessionId, ToolRequest};
 
 use crate::sink::{Flaky, Lines, Switch};
 use crate::tmp::TempDir;