feat(ie-layout): implement CSS flexbox layout #26

Flexbox layout engine (flex.rs):
- flex-direction: row, column, row-reverse, column-reverse
- flex-wrap: nowrap (default), wrap, wrap-reverse
- justify-content: flex-start, flex-end, center, space-between,
  space-around, space-evenly
- align-items: flex-start, flex-end, center, stretch (default)
- Per-item align-self override
- flex-grow: distribute free space proportionally
- flex-shrink: shrink items when space is negative
- flex-basis: explicit base size before grow/shrink
- Flex line splitting for wrap mode
- Recursive child layout within flex items
- Content-based sizing fallback via text measurement

Integration:
- BoxType::Flex variant added
- display:flex routed to layout_flex in box generation
- Block layout dispatches Flex children to flex layout

6 new tests: equal grow, column direction, justify center,
align items center, space-between, wrap.
24 total ie-layout tests.

Author: thomasnemer

crates/ie-layout/src/block.rs

@@ -208,6 +208,16 @@ pub fn layout_block(
                     height: metrics.height,
                 };
             }
+            BoxType::Flex => {
+                crate::flex::layout_flex(
+                    child_idx,
+                    tree,
+                    styles,
+                    content_width,
+                    child_y,
+                    text_measure,
+                );
+            }
             BoxType::Inline | BoxType::InlineBlock => {
                 // Simplified: treat as block for now
                 layout_block(

crates/ie-layout/src/box_generation.rs

@@ -36,7 +36,8 @@ fn generate_boxes(
             }
 
             let box_type = match display {
-                "block" | "list-item" | "table" | "flex" => BoxType::Block,
+                "flex" => BoxType::Flex,
+                "block" | "list-item" | "table" => BoxType::Block,
                 "inline" => BoxType::Inline,
                 "inline-block" => BoxType::InlineBlock,
                 _ => BoxType::Block,
@@ -67,17 +68,17 @@ fn generate_boxes(
             // Insert anonymous block boxes when mixing block+inline children
             let has_block = child_indices
                 .iter()
-                .any(|&i| matches!(tree.boxes[i].box_type, BoxType::Block));
+                .any(|&i| matches!(tree.boxes[i].box_type, BoxType::Block | BoxType::Flex));
             let has_inline = child_indices
                 .iter()
-                .any(|&i| !matches!(tree.boxes[i].box_type, BoxType::Block));
+                .any(|&i| !matches!(tree.boxes[i].box_type, BoxType::Block | BoxType::Flex));
 
             if has_block && has_inline && matches!(box_type, BoxType::Block) {
                 let mut wrapped = Vec::new();
                 let mut inline_run = Vec::new();
 
                 for &idx in &child_indices {
-                    if matches!(tree.boxes[idx].box_type, BoxType::Block) {
+                    if matches!(tree.boxes[idx].box_type, BoxType::Block | BoxType::Flex) {
                         if !inline_run.is_empty() {
                             let anon = LayoutBox {
                                 node_id: None,