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/policy_matching.rs

@@ -8,7 +8,7 @@ use brokerd::policy::{Label, decide};
 use build::{
     allowed, asked, denied, fetch, grant, now, private, read, reason, request, secret, set, shell,
 };
-use proto::{DataClass, DenyReason, Mode};
+use bxcore::{DataClass, DenyReason, Mode};
 
 #[test]
 fn the_most_restrictive_mode_wins_among_three_matching_grants() {
@@ -348,7 +348,7 @@ fn a_decision_and_an_ask_carry_what_the_broker_and_the_runner_need() {
         r#"{"path":"/home/kyle/scratch/a.txt","content":"hello"}"#
     );
     assert_eq!(ask.grant(), "asks");
-    assert_eq!(ask.grant_sha256(), proto::sha256(b"asks").unwrap());
+    assert_eq!(ask.grant_sha256(), bxcore::sha256(b"asks").unwrap());
     assert_eq!(ask.matched_path(), Some("/home/kyle/scratch"));
     assert_eq!(ask.paths(), ["/home/kyle/scratch"]);
     assert!(ask.hosts().is_empty());