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/build.rs

@@ -6,7 +6,7 @@
 
 use brokerd::grants::{GrantSet, LoadedGrant};
 use brokerd::policy::{Ask, Decision, Denial, Outcome, SessionState};
-use proto::{
+use bxcore::{
     CallId, Constraints, DataClass, DenyReason, Grant, Mode, SessionId, Timestamp, ToolRequest,
 };
 
@@ -39,7 +39,7 @@ pub fn grant(id: &str, tool: &str, mode: Mode) -> Build {
             constraints: Constraints::default(),
         },
         // Stands in for the file's hash, and differs from grant to grant.
-        sha256: proto::sha256(id.as_bytes()).unwrap(),
+        sha256: bxcore::sha256(id.as_bytes()).unwrap(),
     })
 }