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/bxcore/tests/fixtures/audit/seq-gap/2026-09-17.jsonl

@@ -0,0 +1,3 @@
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+{"seq":1,"time":"2026-09-17T08:00:01.000Z","prev":"d7a63875b77829169f2c9f1bfb54cf00cc11a7efdcc34fa3a3ea1e889ea38eaa","event":{"type":"result","session":"chat-1","call":1,"decision":0,"status":"failed","class":"private","untrusted":false,"truncated":false,"bytes":25,"sha256":"ed52694265ada1850bdf1a90f9c6b6e1ed9e45fbca44fbe9114639ada3dc8051","taint_after":"private"}}
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