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/loopd/src/selftest.rs

@@ -75,7 +75,7 @@ fn server_matches(client: &Client) -> Result<(), SelfTestError> {
     let expect = &client.config().expect;
 
     let expected = expect.template_sha256;
-    let got = proto::sha256(props.chat_template.as_bytes()).map_err(|_| SelfTestError::Hash)?;
+    let got = bxcore::sha256(props.chat_template.as_bytes()).map_err(|_| SelfTestError::Hash)?;
     if expected != got {
         return Err(SelfTestError::Mismatch {
             what: "chat template sha256",