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/container_grace.rs

@@ -15,8 +15,8 @@ use brokerd::pipes::GRACE;
 use brokerd::policy::{Outcome, SessionState, decide};
 use brokerd::runner::run;
 use build::{grant, now, read, set};
+use bxcore::{Mode, ToolResponse};
 use fake_podman::{Fake, Lines, serial};
-use proto::{Mode, ToolResponse};
 
 fn call(fake: &Fake, extra: &str, log: &Lines) -> (ToolResponse, Duration) {
     let podman = Podman::new(fake.runner(extra), fake.dir.join("egress"), log.sink());