Macro pg_frame 

macro_rules! pg_frame {
    ($buf:expr, $code:expr $(, $op:ident($arg:expr))* $(,)?) => { ... };
}

Encode a framed frontend message with a single-pass write.

The macro computes payload length from a list of primitive write ops, writes the message code + length, then emits payload bytes exactly once.

Supported ops:

• u8(expr), u16(expr), u32(expr)
• i16(expr), i32(expr), i64(expr)
• f32(expr), f64(expr)
• bytes(expr) where expr: impl AsRef<[u8]>
• cstring(expr) where expr: impl AsRef<[u8]>