Process differential E2E advances event time with a future watermark #488

Context

Found while reviewing #478.

The stable process differential creates a data point at wall-clock now and immediately sends an empty watermark at now + 2 seconds:

ddsketch_export(METRIC, sample_ns, &raw_values)
ddsketch_export(METRIC, sample_ns + 2_000_000_000, &[])

This makes the test fast, but it exercises acceptance of future event time rather than the normal collector/backend timing contract. It may hide bugs in real window close, allowed lateness, watermark progression, and query evaluation relative to wall clock. The whole-backend E2E already avoids this by waiting for the controller-selected window to actually end.

Reproduction

./scripts/e2e.sh differential

Location: data_plane/tests/promql_differential_process_e2e.rs, around lines 235-242.

Expected

The stable production-process differential should not require future-dated telemetry.

Preferred options:

1. wait for the one-second window to end, then emit a wall-clock watermark; or
2. expose and wait for an explicit ingest/window-commit signal without modifying event timestamps.

Done when

The test passes using realistic non-future timestamps and deterministically observes the committed window without a fixed blind sleep.