feat(ingest): per-window delta-base rotation for sketch deltas

[zzylol]

Summary

Implements per-window base rotation in the ASAPQuery-backend ingest delta-apply path so that applying per-window DDSketch (and other additive-family) deltas yields per-window reconstructed state instead of an all-time accumulation across windows.

This is the consumer half of the delta–baseline contract (docs/delta-baseline-contract.md §3).

The problem

The OTLP delta-apply path (data_plane/src/drivers/ingest/otel.rs) reconstructed per-series sketch state by merging each window's delta onto a never-reset running base:

state(N) = state(N-1) ⊕ delta(N)

But the edge tumbling window resets per-series sketch state every window, so each window's delta is that window's marginal against an empty base. Accumulating forever therefore over-counts across windows. For HLL it is worse: the register-wise max base has no inverse and holds the all-time max across every window seen, so per-window cardinality queries over-count by orders of magnitude.

The change — per-window base rotation

• Cache entry now carries its window start. The per-series snapshot cache value changes from a bare boxed accumulator to SnapshotCacheEntry { core, window_start } (precompute_engine/ingest_handler.rs).
• Window-boundary detection per series. The delta-apply branch compares the incoming data point's start_time_unix_nano against the window_start stored with the cached base for that series_key. When they differ, the cached base is reset to empty before applying the new window's delta — so within a window deltas accumulate, and at a new window the base starts fresh. state(N) is then window N only.
• Full frames (PROTO_FULL) keep REPLACE semantics and set the stored window_start.
• Sketch-agnostic reset. Adds AggregateCore::reset_to_empty (default no-op) implemented for the additive, delta-capable families — DDSketch, CMS, CountSketch, HLL — each preserving its shape/config (dims / relative accuracy / register width). KLL never deltas, so it never reaches this path.
• The existing "no base yet → drop the delta" guard is unchanged.

Test

Adds delta_apply_rotates_per_series_base_at_window_boundary (drives the real ingest loop): for one series, full(win1) → delta(win1) (same window_start) → delta(win2) (a NEW window_start).

• Within win1 the delta accumulates onto the full frame.
• At the win2 boundary the base is rotated, so the reconstructed state is win2 only (count 20), not win1 + win2 (count 45).

Uses DDSketch (an additive family) so accumulation vs. reset is directly observable. All existing delta tests still pass.

Relationship to other work

• Composes with the edge producer (true per-window deltas against an empty outbound base) in feat(delta): per-window DDSketch delta producer (delta-against-empty) ASAPCollector#456 — the edge emits the window's own sketch as the delta, this PR rotates the backend base to match.
• Builds on chore(ddsketch): consume DDSketch wire format without metric scalars #345 (DDSketch wire format consumed without metric scalars), which landed the DDSketch delta consumer this rotation rides on.
• Unblocks per-window-correct delta queries: per-window finalized state feeds the warm/ASAP-tier per-bucket store, so windowed-range reducers (count_over_time, quantile_over_time, distinct-count over time) aggregate across correct per-window buckets, and the contract's delta-ON == delta-OFF equality gate becomes achievable.

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