refactor(query-engine): centralize range top-k maintenance

[milindsrivastava1997]

Summary

• Centralize required Topk k parsing across the instant-wrapper reference path and refactor(query-engine): unify range pipeline onto output_timestamps + step-major loop (#581 stage E.1-E.3) #635's integrated range pipeline.
• Make malformed Topk metadata fail instead of returning untruncated results.
• Centralize metric-name label formatting and update the stale tie-break comment.
• Rebase the maintenance work onto refactor(query-engine): unify range pipeline onto output_timestamps + step-major loop (#581 stage E.1-E.3) #635's step-major range-pipeline rewrite.

Closes #634

Validation

• cargo test -p query_engine_rust --lib (537 passed, 1 ignored)
• cargo clippy -p query_engine_rust --lib --tests -- -D warnings
• cargo fmt --check

Out of scope

• Binary-expression Topk matching remains tracked in topk(...) as a binary-expr arm: __name__ label mismatch (pre-existing) and metric-name-in-join corruption (#629 Finding 1) #631.
• The selection optimization from PR #629 code review findings #634 finding 8 remains deferred.