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/gatewayd/tests/approvals_text.rs

@@ -1,9 +1,9 @@
 //! The text of an approval's posts and of its ending line (M4b spec, section 6). Do not edit.
 
+use bxcore::{CallId, DataClass, DenyReason, PendingApproval, SessionId, Timestamp, sha256};
 use gatewayd::approvals::text::{
     Ended, MAX_PART_POST, MAX_PARTS, PART_CHARS, code, ended_line, fence, render, with_ending,
 };
-use proto::{CallId, DataClass, DenyReason, PendingApproval, SessionId, Timestamp, sha256};
 
 /// The escape for one code point, built here so this file holds no such character itself.
 fn esc(code: u32) -> String {