Function counter_id_to_symbol 

pub fn counter_id_to_symbol(counter_id: &str) -> String

Convert a counter_id (e.g. ST/US/TSLA, ETF/US/SPY, IX/US/.DJI, ST/HK/700) back to a display symbol (e.g. TSLA.US, SPY.US, .DJI.US, 700.HK).

US index counter IDs (IX/US/...) preserve the leading dot in the code part (e.g. IX/US/.DJI → .DJI.US).