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/approvals.rs

@@ -9,7 +9,7 @@ use std::sync::{Arc, Barrier};
 use brokerd::approvals::{Table, Verdict};
 use brokerd::policy::{Ask, Outcome, SessionState, decide};
 use build::{grant, now, read, set, ts};
-use proto::{CallId, DataClass, DenyReason, Mode, PendingApproval, SessionId, Timestamp};
+use bxcore::{CallId, DataClass, DenyReason, Mode, PendingApproval, SessionId, Timestamp};
 
 fn ask() -> Ask {
     let grants = set(vec![grant("n", "read_file", Mode::Ask).paths(&["/n"])]);