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/tests/http.rs

@@ -56,7 +56,7 @@ fn reads_a_recorded_chunked_response() {
     // The hash recorded in docs/inference-contract.md. If the de-chunking were off by a byte,
     // this would not match.
     assert_eq!(
-        proto::sha256(template.as_bytes()).unwrap().to_hex(),
+        bxcore::sha256(template.as_bytes()).unwrap().to_hex(),
         "f55f52930aa8bf44ab5cb85f99370fcc3c56e9a85640b812086d5330bce5d86b"
     );
 }