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/selftest.rs

@@ -84,10 +84,10 @@ fn the_main_slot_comes_from_the_config() {
 
 #[test]
 fn each_expected_value_is_checked() {
-    let zero = proto::Hash32::ZERO;
+    let zero = bxcore::Hash32::ZERO;
     let cases: [(&str, Edit); 3] = [
         ("chat template sha256", |c| {
-            c.expect.template_sha256 = proto::Hash32::ZERO
+            c.expect.template_sha256 = bxcore::Hash32::ZERO
         }),
         ("context per slot", |c| c.expect.n_ctx = 4096),
         ("slot count", |c| c.expect.slots = 3),