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.rs

@@ -16,7 +16,7 @@ use std::panic::AssertUnwindSafe;
 use brokerd::ledger::{Call, Decided, Grants, NOT_RECORDED, POISONED, STOPPED};
 use brokerd::policy::{Label, SessionState};
 use build::{grant, now, read, set};
-use proto::{
+use bxcore::{
     AuditEvent, CallId, DataClass, DecisionRecord, DenyReason, GrantProblem, Mode, ResultStatus,
     SessionId, ToolResponse,
 };
@@ -80,7 +80,7 @@ fn an_allowed_call_is_recorded_in_full() {
             arguments,
             outcome: DecisionRecord::Allowed {},
             grant: Some("n".to_string()),
-            grant_sha256: Some(proto::sha256(b"n").unwrap()),
+            grant_sha256: Some(bxcore::sha256(b"n").unwrap()),
             taint: DataClass::Private,
             untrusted: false,
         }
@@ -270,7 +270,7 @@ fn a_result_raises_the_state_then_is_recorded() {
             untrusted: true,
             truncated: false,
             bytes: 3,
-            sha256: proto::sha256(b"key").unwrap(),
+            sha256: bxcore::sha256(b"key").unwrap(),
             taint_after: DataClass::Secret,
         }]
     );
@@ -327,7 +327,7 @@ fn a_failure_changes_no_state_and_is_recorded_by_its_message() {
                 "the label the result would have had"
             );
             assert_eq!(*bytes, 18);
-            assert_eq!(*sha256, proto::sha256(b"the tool timed out").unwrap());
+            assert_eq!(*sha256, bxcore::sha256(b"the tool timed out").unwrap());
             assert_eq!(*taint_after, DataClass::Private, "the taint before");
         }
         other => panic!("{other:?}"),