refactor(legacy_expr): step γ3 — WindowedAgg un-fusion bridge

[zzylol]

Summary

Per design.md §3 + §6 "What was removed" row 2: "One canonical form per plan — no WindowedAgg (use Window over Aggregate)".

Follows γ1's strategy (c): keep legacy QueryExpr::WindowedAgg in place; provide a canonical-view helper consumers call on demand. Window-sketch lifecycle invariant preserved — no construction-site rewrites.

New: windowed_agg_bridge.rs (567 lines, 12 tests)

pub fn bridge_windowed_agg_to_canonical(
    agg: &AggIntent, window: &WindowSpec,
    col: &LegacyColumnRef, parent_schema: &Schema,
) -> Result<BridgedWindowedAgg, BridgeError>;

pub fn output_schema_for_windowed_agg(
    input: &Schema, by: &[ColumnId], aggs: &[AggIntent],
) -> Schema;

BridgedWindowedAgg { window_kind, window_size, window_slide, inner: BridgedAggregate } — stacks γ1's BridgedAggregate as the inner Aggregate layer.

Legacy Tumbling/Sliding/Session → canonical WindowKind. Unbounded/Landmark defer via BridgeError::UnsupportedWindowKind.

File placement: separate module

Rationale: WindowedAgg fuses TWO canonical layers (Window over Aggregate). Mixing single-layer + stacked-layer bridges in one file dilutes each surface. γ1's BridgedAggregate is pub-imported as a field — no duplication.

Demo wire

physical/allocator.rs::alloc_node WindowedAgg arm calls the bridge after recursing into input. Appends "canonical view: Window(kind=…, size=…, slide=…) over Aggregate(by_len=…, intent=…)" to NodeAnnotation.rationale. Legacy emit + lifecycle unchanged.

Build + test

• cargo build --release -p controller clean
• cargo test -p controller --lib — 700 passed (was 688 post-γ1; +12 new tests)
• cargo test -p controller --bin controller — 27 passed

Diff: 3 files, +627 / -2

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