refactor(data_plane): rekey backfill processor + SketchStore from agg_id to sid (B7.7)

[zzylol]

Scope

Third sub-step of schema-retirement #5 (issue #272), following B7.6's
ingest-side rekey. The backfill processor's per-window grouping is
now keyed by sid: u64 instead of group_key: String, matching B7.6's
sid-bucketed worker state. The SketchStore exposes a new sid-direct
write path (ingest_precompute_with_sid) so callers that already hold
the bucket sid skip the resolver round-trip inside the mint-driven
ingest_precompute_for_agg_config wrapper.

Sites rekeyed

data_plane/src/storage_engines/sketch_db/backfill/processor.rs

• process_window now groups raw samples into a HashMap<u64, SidBucket> (sid-keyed) instead of HashMap<String, Vec<RawSample>> (group_key-keyed).
• New helper resolve_backfill_bucket_sid mirrors resolve_bucket_sid_for_agg_config from drivers/ingest/otel.rs so backfill and live ingest mint the SAME sid for the same (metric, grouping-values, agg_kind) tuple. This invariant is what lets backfill writes land in the same store row live ingest already populated for [created_at, ∞).
• Per-bucket writes go through the new ingest_precompute_with_sid path; the mint-driven sibling is no longer called from this file.
• Resolver-less fallback (legacy / registry-only test setups) keeps a stable per-group_key bucket id so accumulator builds still preserve sample ordering — but the write itself is skipped in that branch anyway (no resolver ⇒ no precompute write, matching pre-B7.7 behaviour).

data_plane/src/storage_engines/sketch_db/index/mod.rs

• New pub fn ingest_precompute_with_sid(sid, agg_cfg, output, accumulator) takes the bucket sid directly. The existing ingest_precompute_for_agg_config is refactored into a thin mint-driven wrapper that delegates to the new entry point — callers that don't yet hold the sid (the live SketchStoreSink) keep working unchanged.
• Extracted build_attrs_fp_and_label_map shared by both methods so the mint-driven path (B7.6) and the sid-direct path (B7.7) stay byte-identical on the values they hand to the index.

Shape changes

• BackfillWindowProcessor::process_window internal bucket map: HashMap<String, Vec<RawSample>> → HashMap<u64, SidBucket { group_key, samples }>.
• New public method SketchStore::ingest_precompute_with_sid(sid: u64, &AggregationConfig, &PrecomputedOutput, &dyn AggregateCore) -> Option<u64>.
• SketchStore::ingest_precompute_for_agg_config body collapsed to a sid-mint then delegate; external contract unchanged.

Files touched

• data_plane/src/storage_engines/sketch_db/backfill/processor.rs (+283/-37)
• data_plane/src/storage_engines/sketch_db/index/mod.rs (+91/-39)

Tests added

• process_window_buckets_by_sid_via_resolver — drives process_window end-to-end with two distinct svc values × two samples each, asserts exactly two sids land in the SketchStore, both classify() as Hit, and registry provenance is one entry per window.
• backfill_sid_matches_live_ingest_sid_for_same_grouping_values — locks the live-vs-backfill sid namespace invariant: the sid the backfill helper computes for (cfg, \"latency{svc=a,zone=z0}\") must equal what the live ingest path's resolve_bucket_sid_for_agg_config mirror computes for the same (metric, grouping-values, agg_kind) tuple via the SAME resolver.

Not in scope (deferred follow-ups)

• output_sink.rs production code already consumes output.policy_fp (PR refactor(data_plane): rekey ingest bucketing from (agg_id, group_key) to sid (B7.6) #284's report: "no changes needed there"). Its only remaining aggregation_id() site is in a test that builds a StreamingConfig map keyed by policy_fp.as_u64() — the legitimate policy-registry use, not a bucket key.
• worker.rs / series_router.rs / drivers/ingest/otel.rs are B7.6's domain (already merged) — not touched.
• Remaining aggregation_id() accessor sites are all test-side StreamingConfig map-key uses (the map IS keyed by policy_fp.as_u64()) — those stay until the accessor itself is retired after docs: design for SimpleMapStore persistence (mem limit + disk flush) #4 (re-enable ignored tests).

Test plan

• cargo build -p data_plane — clean (only pre-existing warnings)
• cargo test -p data_plane --lib — 715 passed / 0 failed / 2 ignored, no regressions vs origin/main
• cargo test -p data_plane integration suite — same 2 pre-existing failures controller_plan_to_query_full_roundtrip_ddsketch / _kll that PR refactor(data_plane): rekey ingest bucketing from (agg_id, group_key) to sid (B7.6) #284 confirmed are pre-existing on a fresh origin/main worktree. Unrelated to B7.7.
• Both new regression tests pass

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