refactor(control_plane): unify Replanner through typed cumulative emit; retire generate_streaming_config_yaml (Option B)

[zzylol]

Summary

Pre-task the control plane had two emit paths into the backend's /api/v1/streaming-config:

1. Typed cumulative path (post PR fix(emit): cumulative streaming-config across (metric, role) pairs (B2 follow-up) #287) — handle_plan runs planner → BackendStageConfig → cache insert by (metric, role) → cumulative POST. Correct under the data plane's atomic handle.swap(new_config) swap.
2. Legacy single-aggregation path — Replanner::replan_metric_role → generate_streaming_config_yaml → single-aggregation YAML POST. Under the same swap, this WIPED the cumulative state every time plan-expiry or accuracy-violation fired it. The smoke harness never triggered the latent landmine because replan_metric_role ran only on SLA-violation / plan-expiry, neither of which fires in the 90s soak.

This PR is Option B of the task: collapse to ONE emit path and retire the legacy single-aggregation YAML.

Changes

• New helper emit::backend_push::post_typed_backend_for_role — single entrypoint for every typed cumulative POST. Updates the shared (metric, role) → BackendStageConfig cache, builds the cumulative streaming-config + per-metric merged storage-routing, fire-and-forget POSTs both. Used by both handle_plan and Replanner so both writers accumulate against the same state.
• handle_plan Backend arm — ~120 lines of inlined cumulative-emit / cache plumbing replaced with a single call to the helper.
• Replanner::replan_metric_role — drops the legacy generate_streaming_config_yaml + push_or_log call, runs the same planner → stage-split → Backend extraction as handle_plan (new Replanner::build_backend_stage_config private helper), then POSTs through the typed helper. Now correctly cumulative on plan-expiry / SLA-violation triggers.
• Legacy YAML emitter retired — generate_streaming_config_yaml and the entire emit::asapquery_backend module (287 lines) deleted. Comments in emit::stage_config and emit::mod updated to document the retirement.
• Startup replan tick — Replanner::replan_all() runs immediately after the workload-registry pre-pop loop and BEFORE the HTTP server binds, so the backend's cumulative streaming-config + storage-routing are populated from t=0 (the smoke harness never POSTs /api/v1/plan, so without this the data plane's static backend-streaming.yaml was the only source of truth).
• OpAMP on-connect replan tick — defensive idempotent re-fire of replan_all() on every agent connect; guards against start-order races where the backend came up after the startup tick.
• ExactAgg(Sum) fallback — the typed bind_workload_typed binder declines for raw-passthrough sum/rate/count workloads. Pre-Option B these metrics relied on the static backend-streaming.yaml having a DDSketch entry that the broken legacy emit happened not to overwrite. Post-Option B the cumulative POST has authoritative final say, so we extend BackendAggregation with agg_type_override: Option<String> and synthesize a BackendStageConfig carrying agg_type_override: Some(\"Sum\") when the typed binder declines for a Sum/Count role. build_backend_aggregation_json honours the override by emitting aggregationType: \"Sum\" and parameters: {}. The cumulative streaming-config now carries an ExactAgg(Sum) row for http_requests_total alongside the 5 sketched metrics.

Smoke test (90s soak) — before / after

Sub-check	Pre-PR	Post-PR
USE_TYPED_STAGE_SPLIT.*posting typed backend JSON log count	0	7
/api/v1/streaming-config aggregation count	2 (static yaml only — would be wiped on first replan)	6 (5 sketched + Sum for http_requests_total)
quantile_over_time(0.99, http_requests_total_latency_ms[5m])	DDSketch result	DDSketch result (no regression)
Aggregation count after 10s late-replan check	static yaml could be wiped	6 (cumulative state persists — the Option A landmine that this PR avoided by retiring the legacy path)
Backend's db/schemas for http_requests_total	absent	4 ExactAgg(Sum) sids (one per zone) registered

Known limitation (out of scope)

sum by (zone) (http_requests_total) still returns No result for query because the data plane's PromQL evaluator does not yet dispatch sum by (...) instant queries to the ExactAgg sids it has registered. The streaming-config now correctly surfaces the metric to the data plane (verified: /api/v1/db/schemas returns 4 ExactAgg(Sum) sids for http_requests_total); closing the query-side gap is a data_plane/ workstream this PR explicitly honours the DO NOT touch data_plane/ constraint on. The control-plane side of the Option B unification is complete.

Test plan

• cargo build -p control_plane clean
• cargo test -p control_plane --lib — 738 pass (baseline maintained; 2 tests removed alongside the retired emitter)
• cargo test -p control_plane --bins — 28 pass
• rg \"generate_streaming_config_yaml\\(\" control_plane/src/ returns 0 hits (no callers remain)
• Smoke test sub-checks 1 (USE_TYPED_STAGE_SPLIT log), 2 (cumulative aggregations include Sum), 4 (quantile no regression), 5 (10s late-replan idempotent) all pass.
• Smoke test sub-check 3 (sum by (zone) returns per-zone results) still fails — data plane Sum-capability gap noted above.

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