Support sliding window candidates in the query engine (execution)

[milindsrivastava1997]

Sub-issue of #554.

Scope: actually serving queries correctly against sliding-window aggregations — the execution-side work, separate from generating valid candidates (#554's other sub-issue, #556).

Concretely:

• Relax capability_matching.rs's window_compatible() Sliding rule beyond exact-match (data_range_ms == window_size_ms) to allow data_range_ms a multiple of window_size_ms, mirroring Tumbling's existing rule.
• Implement merge (mergeable sketch types) / subtract (subtractable types) across multiple stored sliding-window buckets in simple_engine's query-serving path (create_store_query_plan/execute_and_merge_store_queries in simple_engine/mod.rs, which today unconditionally does a single exact-match lookup for Sliding and explicitly skips merging).
• Update planner/cleanup.rs's WindowType::Sliding branch to be window-count-aware instead of hardcoding read_count_threshold = 1.
• Resolve the epoch/slide-interval alignment question for exact-match lookups (align_end_timestamp_promql only aligns to the scrape interval, not the aggregation's slide_interval_ms).
• Owns the end-to-end correctness test (plan → run → query a sliding-window aggregation → assert correct results) — Manual windowing override (tumbling/sliding) in config.yaml #555 explicitly defers this test here.

Needs its own design session — not scoped further here. See #554 and the design docs linked there for context.