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>

scripts/check-crate-deps.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
-# Fails if `proto` depends on any workspace crate, or if any other crate depends on
-# a workspace crate other than `proto`. Every manifest section whose name contains
+# Fails if `bxcore` depends on any workspace crate, or if any other crate depends on
+# a workspace crate other than `bxcore`. Every manifest section whose name contains
 # "dependencies" (including [dev-dependencies] and [build-dependencies]) is
 # inspected. A dependency name can appear as a plain key (`x = "1"`), a dotted key
 # (`x.workspace = true` or `x.path = ".."`) or a table header (`[dependencies.x]`);
@@ -69,10 +69,10 @@ for crate in "$ROOT"/crates/*/; do
             if [ "$key" = "$w" ]; then hit=1; break; fi
         done
         if [ "$hit" -eq 1 ]; then
-            if [ "$crate_name" = "proto" ]; then
+            if [ "$crate_name" = "bxcore" ]; then
                 echo "check-crate-deps: $crate_name depends on workspace crate $key" >&2
                 status=1
-            elif [ "$key" != "proto" ]; then
+            elif [ "$key" != "bxcore" ]; then
                 echo "check-crate-deps: $crate_name depends on workspace crate $key" >&2
                 status=1
             fi