Function format_source_grouped 

pub fn format_source_grouped(source: &str, style: GroupingStyle<'_>) -> String

format_source, with the thousands-separator rule supplied by the caller.

The style comes from the LEDGER, not from the tool: option "render_commas" and per-commodity render_commas:, resolved by whoever holds the options (the CLI after a load, the LSP from its ledger state, the FFI session from its own). That is what keeps this a canonical form rather than a knob — the output is still a function of the input, the declaration simply travels with it.

format_source passes GroupingStyle::default, so every existing caller and every ledger that has not opted in is byte-for-byte unaffected.