compaction-counter: hardcoded reclaimed-tokens arrow is not overridable like other widget symbols

[zachthedev]

formatReclaimedSuffix in src/widgets/CompactionCounter.ts hardcodes a leading down-arrow (U+2193) before the reclaimed-token count shown by showReclaimed. After #431 added per-widget symbol overrides, this is the one widget glyph with no character/slot override path.

Request: expose the reclaimed prefix as an overridable symbol slot (default U+2193), consistent with the git/jj widget symbols.

Secondary: formatTriggerSuffix wraps the trigger split in parentheses; a separator-based layout would read more consistently next to other widgets.