Clarify ERB escape allocation alert as a false positive

[Copilot]

• Assessment: The reported alloca-in-loop alert is a false positive. ALLOCV_N runs only when buf is null; subsequent loop iterations reuse the allocated buffer.
• Code changes: None. The alert does not indicate repeated stack allocation.

if (!buf) {
    buf = ALLOCV_N(char, vbuf, escaped_length(str));
}