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/src/sessions.rs

@@ -3,7 +3,7 @@
 
 use std::collections::{BTreeSet, HashMap};
 
-use proto::SessionId;
+use bxcore::SessionId;
 
 use crate::mm::Post;
 
@@ -71,7 +71,7 @@ pub enum ApprovalCommand {
 }
 
 /// The longest reason a `!deny` may give, in characters.
-pub const MAX_REASON: usize = proto::MAX_REASON;
+pub const MAX_REASON: usize = bxcore::MAX_REASON;
 
 /// `!approve …` or `!deny …` as an approval command; `None` for any other text.
 pub fn approval_command(text: &str) -> Option<ApprovalCommand> {