Allow independent formatting of composite sub-values

[nh13]

The existing format string system is great, but some modules fuse multiple values into a single $value that can't be independently controlled:

1. context_bar: $value = ███░░░35% — can't strip the 35% off
2. used_tokens: $value = 36%(73k/200k) — can't insert a space into the middle
3. total_duration_ms: $value = 248059 — can't convert ms to 4m8s

Splitting these into separate placeholders (e.g. $bar, $pct, $used, $total, $hours, $minutes) would let users compose output freely. Happy to submit a PR if this direction sounds good.