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>

Cargo.toml

@@ -1,7 +1,7 @@
 [workspace]
 resolver = "3"
 members = [
-    "crates/proto",
+    "crates/bxcore",
     "crates/loopd",
     "crates/brokerd",
     "crates/gatewayd",
@@ -20,7 +20,7 @@ publish = false
 unsafe_code = "forbid"
 
 [workspace.dependencies]
-proto = { path = "crates/proto" }
+bxcore = { path = "crates/bxcore" }
 serde = { version = "1.0.229", features = ["derive"] }
 serde_json = "1.0.151"
 humantime = "2.4.0"