feat(planner): Pass 2 identical-expression rule (shared-input variants)

[zzylol]

Rebased on main d4869a7 (DF 54).

Why

#509 Stage 1 is Pass 1 (local alternatives) + Pass 2 (ASAP-aware CSE). Pass 2's identical-expression rule was missing. In Example 1 both queries read http_requests_total[1m], so the doc's "Raw with a shared input" plan was never a candidate. The facade also called share_common_sub_dags unconditionally while the devtool never did, so the two disagreed. In practice the facade's merge was a no-op for PromQL: the scan had no unique key, and CSE's legality rule refuses to share without one. Part of #580; stacked on #587 (#572).

What

• Pass 2 identical-expression rule (asap-logical-optimizer/src/pass2/identical_expressions.rs). Stage 1 returns sharing variants: Pass 1 over the queries as written, and Pass 1 over the queries with identical sub-DAGs merged by the existing share_common_sub_dags. The shared variant is added only when the merge removes nodes. Shared candidates are labelled "… · shared input" and numbered after the independent ones.
• Selection treats the variant as an outer choice (asap-plan-selection). The per-target DP runs once per variant, and the cheapest wins. In the shared variant, identical summary producers are also merged after composition. They are priced the same way in the DP, in exhaustive enumeration and in the final full check. The coupling guard also checks pairs of targets that read a common input. Fallback and the final full check apply per variant.
• One library path. plan_selection::plan_stages runs Stage 1 (Pass 1 + Pass 2) → Stage 2 → Stage 3. The facade calls it and no longer pre-merges. The devtool calls it and only serializes the result (with display set so every candidate is priced).
• Scan key. with_promql_series_identity now declares (series identity, ts) as the identified PromQL scan's unique key. A series has at most one sample per timestamp. CSE's legality rule needs this key to share the range selector.

How it was checked

• DP = exhaustive on Example 1 (48 combinations, P44) and on a new workload where sharing merges a whole target: sum by (job) (rate(x[1m])) and max by (job) (rate(x[1m])), 16 + 8 combinations, where a shared candidate wins.
• Un-ignored stage1_keeps_independent_and_shared_variants and stage3_shared_input_is_not_costlier. Hydra was dropped from both loops; it stays covered by the still-ignored stage1_q2_summary_families_are_heap_sketches_and_hydra, because Pass 1 has no Hydra alternative.
• New stage3_selects_a_shared_input_plan: the selected plan shares the input, and it saves exactly one scan plus one range node.
• Acceptance spec, its 48-row table and the viewer fixture are regenerated from the planner's output.

Before this PR (Example 1)

1 → 24 → 24 → 1. Selected P20 "Q1 exact (Sum acc, Rate acc) · Q2 exact (Sum acc)", 79.401 cpu ms. The scan (23.2) and range (4.0) are priced twice, once per query.

After this PR (Example 1)

1 → 48 → 48 → 1 (24 independent + 24 shared; 16 Count-Min + heap candidates invalid as before). Selected P44 "Q1 exact (Sum acc, Rate acc) · Q2 exact (Sum acc) · shared input", 52.201 cpu ms. The same choices as P20, with one scan → range:

Node	P20	P44
scan 4,000,000 samples	23.2 × 2	23.2
time range 60 s	4.0 × 2	4.0
Rate acc build + finalize (Q1)	5.0	5.0
Sum acc build + finalize (Q1)	1.0	1.0
Sum acc build + finalize (Q2)	5.0	5.0
sort 1M rows / 100 partitions + limit (Q2)	14.001	14.001
Total	79.401	52.201

The best CountSketch + heap plan is P48 at 164.201 (shared).

Gate

cargo fmt --all --check ✓; cargo clippy --workspace --all-targets --all-features -- -D warnings ✓; cargo test --workspace: 1,514 passed / 10 ignored (#587: 1,508 / 12). The difference is 4 new tests (2 Pass 2 unit tests, shared_target_dp_equals_exhaustive, stage3_selects_a_shared_input_plan) and 2 un-ignored. Viewer: python3 -m unittest discover -s tools/dag-viewer -p test_render.py, 29 OK (6 skipped).

Refs #509, #580, #572.

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