feat: list current-series TopK heap alternatives in PlanSpace

[zzylol]

Stacked on #483.

Why

PlanSpace decides what to compute. It does not decide placement. The summary
maintenance lifecycle sets each node's timing (precompute or query), and the
physical compiler reads that timing. An instant-vector TopK has heap
realizations (CMS/CountSketch with heap) that rank rows carrying the complete
PromQL series identity. These are real logical alternatives, but PlanSpace did
not list them, so the backend built them outside Planner.

What

• ReplacementStrategy::propose_for_root (new, default: none): whole-query
  logical alternatives for a targeted workload root. search_workload_with_targets
  asks it once per distinct root. It exists because these candidates need input
  rows (the series identity) that the logical root does not expose, so the
  per-subtree propose cannot produce them.
• SketchAlgorithmStrategy implements it for current-series TopK only. It
  resolves identity-carrying rows with with_promql_series_identity (moved into
  asap-types so Planner can call it), then finalizes and deduplicates the
  candidates.
• ReplacementProvenance::RootPhysicalRealization marks these candidates so that
  global_selection never commits them. Deployment validates and prices them.
• Fixed-window and query-time Rate aggregation are placement variants, so they
  are not listed. fixed_window_rate_candidates and
  query_time_rate_aggregation_candidates stay public for the backend's
  current direct callers. Lifecycle-assigned timing replaces them in a later PR.

Before this PR

enumerate_candidate_dags_for_root for topk by(job)(1, m) lists only
logical-root candidates. To see the current-series heap, the backend applies the
series-identity pass and calls current_series_topk_candidates itself.

After this PR

The same call also lists finalized current-series heap readouts, each
bindable with promql_rows::compile_current_series_readout. The inventory for
sum by(job)(rate(m[1m])), topk by(job)(2, rate(m[1m])), rate(m[1m]) and
unrelated queries is unchanged, and so is default selection.

Validation

• crates/asap-physical-operators/tests/planspace_series_identity_heap.rs:
  current-series heaps are listed and bindable. Rate placement variants are not
  listed (this test fails on the earlier feat: list current-series TopK heap alternatives in PlanSpace #477 head). Unrelated inventories are
  unchanged. Global selection never commits a heap. Repeated roots are proposed
  once.
• cargo fmt --check, cargo clippy --workspace --all-targets -D warnings,
  cargo test --workspace --no-fail-fast. The only failure is the existing
  devtools dag_export::workload_node_ids_make_smaller_topk_reuse_explicit.

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