Replaces {expr} tokens in template by evaluating expr in envir.
Used in .report_api_error() / .report_api_warning() so callers can
write hints like "No data for {game_id}" and have {game_id} resolve
against the function's frame at the call-site.
.interp_braces(template, envir = parent.frame())character(1).
character(1).
environment to evaluate expressions against.
Per-token failures (unbound name, parse error) leave the literal
{expr} in place rather than erroring, so partial interpolation still
produces a useful message.