Function synthesize_cell_binding_name 

pub fn synthesize_cell_binding_name(
    metric_name: &str,
    dimension: &str,
) -> String

The kernel wire a metric’s cell COORDINATE is lowered to, per (metric, dimension).

Keyed by metric as well as dimension because two metrics in one scope may place into the same dimension by different expressions — bytes_out from one captured column and bytes_in from another. A per-dimension name would collide and one placement would silently win.

Same __ convention as synthesize_metric_binding_name: it cannot collide with an author-declared output, and diagnostics can filter it as internal.