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/serve_runner.rs

@@ -10,11 +10,11 @@ use std::path::Path;
 use std::process::{Child, Command, Stdio};
 use std::time::{Duration, Instant};
 
-use fake_podman::{Fake, IMAGE, serial};
-use proto::{
+use bxcore::{
     CallId, DataClass, Envelope, Message, PROTOCOL_VERSION, SessionId, ToolRequest, ToolResponse,
     read_frame, write_frame,
 };
+use fake_podman::{Fake, IMAGE, serial};
 
 struct Running(Child);