refactor: leave precompute/query placement to the lifecycle layer

[zzylol]

Stacked on #482.

Why

The agreed layering says logical PlanSpace decides what to compute and a
chosen lifecycle assignment decides timing. Two logical-side sites still chose
placement for grouped Rate→Sum and Rate heap Top-K:

• SketchAlgorithmStrategy::{fixed_window_rate_candidates, query_time_rate_aggregation_candidates}
  (feat: expose physical-ready logical candidates in Planner selection #472) returned the ordinary candidate with Rate finalization flipped between
  ingestion and query time: timing-only duplicates.
• DAG assembly replaced a selected grouped Sum over an inner aggregate with a
  query-time exact Sum, so the Sum state never reached the inventory. This is why
  feat: derive execution timing from a summary lifecycle plan #482 found Candidate B (Sum in precompute) not expressible as a lifecycle
  choice.

What

• Remove both placement-variant methods.
• Assembly keeps a selected Sum summary that realizes its inner aggregate. The
  query-time residual (fix(sql): preserve nested temporal aggregates #372) still applies when the outer summary would hide the
  inner aggregate in KeepPreAsap.
• promql_rows::compile_fixed_window_rate_aggregation now takes the
  lifecycle-timed PostAsapDag.
• Comments mark the remaining strategy timings as initial layout; design docs
  describe Candidates A/B as lifecycle choices.

Before this PR

sum by(job)(rate(m[1m])) inventory: raw | Rate state → query-time exact Sum
Sum in precompute: only via fixed_window_rate_candidates (timing-flipped copy)

After this PR

inventory: raw | Rate state → query-time exact Sum | Rate state → Rate readout → Sum state
Rate CM + Sum CM        → precompute: Rate + Sum builds;  query: Sum readout
Rate CM + Sum Ephemeral → precompute: Rate build;         query: Rate readout → Sum build → readout

Default global_selection for grouped Rate→Sum now returns the Sum-state
candidate instead of the query-time residual. precompute_candidates reads its
query root (assemble_selected_query) and finds the Rate readout by edge. Every
other fixture is unchanged.

Not in scope

• Maintained populations (MaintainPopulation): execution_timed_dag refuses
  them until lifecycle enumeration covers non-SummaryAgg state.
• Deriving the precompute frontier from timing inside the physical planner.
  Explicit compile_candidates frontiers can still persist per-series rate
  values.
• The lifecycle layer does not yet tell a per-window rebuild apart from
  incremental maintenance for rate-snapshot heaps/Sums. The comment states the
  "no accumulation across evaluations" requirement.
• The query-time exact Sum residual and Sum state + Ephemeral give the same
  placement through different operators. Both remain as logical alternatives.

Validation

• Written first and failing before the fix:
  grouped_rate_sum_inventory_keeps_sum_state_for_lifecycle_placement (unit) and
  grouped_rate_sum_placement_is_a_lifecycle_choice (e2e, both placements
  compiled to the expected precompute/query contents).
• Guard: rate_candidate_inventories_have_no_timing_only_duplicates.
• maintained_rate_heap_lifecycle_compiles_fixed_window_precompute replaces the
  fixed-window heap test and executes across the state boundary.
• cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings,
  cargo test --workspace --no-fail-fast: 1372 passed, 0 failed.

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