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/ledger_answer.rs

@@ -14,7 +14,7 @@ use brokerd::approvals::Verdict;
 use brokerd::ledger::{Answer, Answered, Decided, Grants, Ledger};
 use brokerd::policy::{Ask, Label};
 use build::{grant, now, read, set};
-use proto::{
+use bxcore::{
     ApprovalAnswer, AuditEvent, CallId, DataClass, DecisionRecord, DenyReason, Mode, SessionId,
 };
 use rig::Rig;