fix(gbrain-lib): pin LC_ALL=C in varname validator (macOS locale guard)

In many macOS shells the default locale (e.g. en_US.UTF-8) makes bash
glob brackets like `[A-Z]` match lowercase letters too, so the existing
`case "$name" in [A-Z_][A-Z0-9_]*)` branch lets names like `lower-case`
through validation. The function then trips `printf -v "$varname"` and
`export "$varname"` with `not a valid identifier` errors that surface
mid-prompt, which is exactly what the validator was supposed to prevent.

Pinning `LC_ALL=C` inside the function gives ASCII-only bracket semantics
on both macOS and Linux, matching the documented `[A-Z_][A-Z0-9_]*`
contract. Declared `local` so it doesn't leak to the calling shell —
`gstack-gbrain-lib.sh` is documented as a sourced helper, so a bare
assignment would mutate the caller's locale for the rest of the process
(silently affecting downstream `sort`, `tr`, locale-aware globs in the
same shell, etc.).

The existing regression test
`test/gbrain-lib-verify.test.ts:'rejects invalid var names'`
already covers the macOS repro shape (passes `lower-case` and expects
the validator to reject + emit `invalid var name`). On Linux CI the
test silently passed because `LC_ALL=C` is the typical default; on
macOS dev boxes it fails.

Verified:
- `bun test test/gbrain-lib-verify.test.ts`: 22 pass, 0 fail (on macOS).
- `_gstack_gbrain_validate_varname lower-case; echo $?` → 2.
- `_gstack_gbrain_validate_varname FOO_BAR; echo $?` → 0.
- Caller's LC_ALL preserved across calls (confirmed via sourced bash).

bin/gstack-gbrain-lib.sh

@@ -27,8 +27,22 @@
 # restore), D16 (pooler URL paste hygiene with redacted preview).
 
 # _gstack_gbrain_validate_varname <name> — returns 0 if usable, 2 otherwise.
+# `local LC_ALL=C` is load-bearing twice over:
+#   1. In many macOS shells the default locale (e.g. en_US.UTF-8) makes `case`
+#      glob brackets like `[A-Z]` match lowercase letters too. Without the
+#      LC_ALL=C pin, names like `lower-case` pass validation and then trip
+#      `printf -v "$varname"` and `export "$varname"` with "not a valid
+#      identifier" errors the caller can't easily distinguish from other
+#      failures.
+#   2. `local` is required because this file is documented as a sourced helper
+#      (see header), so a bare `LC_ALL=C` would mutate the caller's locale for
+#      the rest of the process — silently affecting downstream `sort`, `tr`,
+#      and any locale-aware glob in the same shell.
+# Together they give ASCII-only bracket semantics on both macOS and Linux
+# (matching the documented `[A-Z_][A-Z0-9_]*` contract) without leaking.
 _gstack_gbrain_validate_varname() {
   local name="$1"
+  local LC_ALL=C
   case "$name" in
     [A-Z_][A-Z0-9_]*) return 0 ;;
     *) return 2 ;;