planner/optimizer: dedupe PromQL binary-arm decomposition (aqe_extractor.rs vs planner/promql.rs) #413

planner/optimizer — `aqe_extractor.rs`'s `decompose_to_leaves`/`arm_to_query_string`/`strip_parens` (around line 250) duplicates `SingleQueryProcessor::get_binary_arm_queries`/`strip_parens` already in `planner/promql.rs:31-145`.

Two copies of PromQL binary-arm decomposition logic will drift over time. Unifying means picking which module owns the shared logic — worth a short design discussion, not just a blind extraction.

Found during code review of PR #407.