Remove QueryExpr::Window (ψ) — no producer exists, PromQL ranges always collapse to TimeRange

[milindsrivastava1997]

Summary

QueryExpr::Window (crates/ir/src/intent_algebra/query_expr.rs:595) is documented as "tumbling / sliding / session window over the time axis" for streaming query-repetition semantics. Grepped every constructor of it across the workspace (not just consumers/match-arms) — there isn't one.

The only path that could produce it is L2→L3 conversion, and crates/l2/src/lower.rs:353 collapses it unconditionally:

// Standalone L2 Window (bare range vector `m[5m]` not inside a
// recognized single-stat Aggregate): map to TimeRange.
LQueryExpr::Window { duration, input, .. } => CQueryExpr::TimeRange { .. }

Issue #179 independently confirmed the same thing while reviewing convert's match arms: "this mapping is unconditional — convert never produces canonical's streaming Window variant at all, so there's no context-dependent disambiguation happening here, contrary to an earlier assumption in review."

Confirmed empirically too: lowered every query across all 7 corpora (~2600 queries, 4 PromQL + 3 SQL) and walked every resulting tree — Window never appears, and by the above it structurally can't.