1.4.0: runtime health probes + settings-form debt

OpcacheHealthProbeCollector and ApcuHealthProbeCollector are
Shape-3 collectors that snapshot per-runtime stats once per
cron and write to cwv_probes (hit rate, memory used, wasted /
fragmentation, cumulative restarts, entry count). Both off by
default; toggled per-runtime from opcache_probe_enabled and
apcu_probe_enabled, added by cwv_update_10036.

HealthTimeSeriesPanel reads the most recent 30 rows for each
collector key and renders one table per runtime; runtimes with
no probe rows render nothing.

Settings-form debt paid off: every opt-in collector that
landed since 1.0.0 now has form surface — backend cache,
render tree, 1.2.0 upstream-ID headers, 1.3.0 database
instrumentation, edge-cache probe targets / timeout, and the
new runtime probes. Operators on 1.0.0-1.3.0 used drush cset;
1.4.0 is the canonical surface.