feat(charts): quieter chart chrome: solid grid, short time ticks, lighter stacked areas - #2569

🟢 Confidence 10/10 · safe to merge. Only the tick-rule test file changed since the last pass, and I reproduced every new assertion exactly under Node 24 / ICU 78.3.
_{no findings · tests covered · risk medium}

Quieter shared chart chrome: solid hairline grid, shorter 12-hour time ticks, light-edged stacked areas, display-face tooltip values and muted legends. The test file added since the last pass pins the tick rules and holds on the head commit.

• formatTickClock drops a 12-hour tick's minutes on the hour and reads them off an en-US formatter
• Grid renamed DASHED_Y_GRID → Y_GRID and drawn solid across 13 definitions
• Stacked areas fill at 30% under a top edge; bars round only their top

_{b7671cb · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}