TimeSeriesChart: remove DATA MISSING pill labels, keep tinted band only

[Flotapponnier]

The 'MOBULA/CODEX/GECKO DATA MISSING' pill has been a recurring source of confusion: it accuses a specific provider but the underlying guard can't always tell 'aggregator crash' apart from 'our harness reconnect' or 'worker step misalignment on a 30 min silence in a 7 d view'. When the label is wrong, it's actively misleading.

Simplify: drop the pill entirely, keep just the tinted vertical band with dashed edges. Reads as 'no data recorded here' without asserting a cause. When we later add a first-party status signal (e.g. a badge sourced from ws_connected / harness health per-slug), it can live somewhere less exposed than mid-chart.

Changes

• Delete the second all.map that rendered the pill rect + text.
• Delete the slot assignment loop (only mattered for stacked pill labels).
• Drop unused Band fields (name, slot).

Effect

Every existing tinted band stays exactly where it is. The label overlay disappears. Net -70 lines.

Test plan

• typecheck clean
• After deploy: charts on aggregator-head-lag show tinted bands but no pill labels