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/audit.rs

@@ -8,7 +8,7 @@ use std::os::unix::fs::PermissionsExt;
 
 use audit_dir::{D1, D2, TempDir, denied, lines, ts};
 use brokerd::audit::{AuditError, RECOVERED_NOTICE, Writer, verify_dir};
-use proto::{AuditRecord, Hash32, sha256};
+use bxcore::{AuditRecord, Hash32, sha256};
 
 fn mode(path: &std::path::Path) -> u32 {
     std::fs::metadata(path).unwrap().permissions().mode() & 0o777