finish_range_context reads streaming_config twice, risking value/key config inconsistency on hot-reload #598

`finish_range_context` (`asap-query-engine/src/engines/simple_engine/promql.rs`) acquires `self.streaming_config.read()` twice: once for the value aggregation's `window_size_ms` (~line 579-584), once for the key aggregation's, added by #583 (~line 614-621). Each is a separate, momentary lock acquisition — the guard is dropped immediately after `.map(...)`.

`streaming_config: RwLock<Arc>` (`mod.rs:152`) is genuinely hot-swappable via `SimpleEngine::update_streaming_config` (`mod.rs:344-346`), presumably called when the planner recomputes configs. If a swap lands between the two read-lock acquisitions in `finish_range_context`, the value-side and key-side metadata for the same range query could be drawn from two different config generations — inconsistent window sizes, bucket widths, etc.

Before #583 there was only one `.read()` call here, so there was nothing to be inconsistent with. #583 added the second lookup, doubling the exposure window and introducing this specific new risk.

Fix should be simple: acquire the read lock once, look up both configs from the same guard before dropping it.

Found during a code review of #583/#587 (PR #595).