render: real block layout for mixed content, owner-space overflow clips

Two structural fixes to the layout and paint model.

Mixed inline+block content: a block holding both was promoted wholesale
to a flex-row-wrap container to approximate the inline formatting
context. That collapsed contentless block children to width 0 (a
position:relative hero wrapper whose only child is an absolute fill
image vanished with it) and let text share lines with sibling blocks.
Now the parent stays a real block, block-level children stay direct
children (auto width fills the containing block), and each maximal run
of consecutive inline-level siblings becomes one anonymous block-level
box carrying the IFC. A run of pure text and foldable inline wrappers
collapses to a single shaped cosmic-text leaf (one taffy node per run
instead of one per word); runs with atomic inline boxes fall back to a
flex-wrap wrapper. Absolutely positioned children neither join nor
break runs and no longer count as inline content, so their containing
block keeps its real used width and taffy resolves width:100%/inset:0
fills against it.

Overflow clips: an inherited overflow:hidden clip was shifted by the
painted node's own accumulated translate, so a carousel track's
translateX dragged the viewport's clip along with every slide and all
slides stayed visible stacked. Clips are now stored in the clip owner's
coordinate space (owner translate baked in at resolve time) and no
longer move with descendants, for boxes, per-word text, and shaped
inline items alike. The same walk records per-node accumulated
translates once, replacing the per-painted-node ancestor walks. Also
parse translate3d (Swiper-style carousels position their track
exclusively through it) and make transform:none reset a lower-priority
transform.

Verified: abs-fill repros that rendered 0x0 now fill across
block/flex/grid parents; carousel repro shows exactly one slide;
danluu.com mismatch vs Chromium 13.0 to 5.7 percent; no regressions
across the prose and framework spot sets; obstacle course 33/33; 36
percent faster on a layout-bound synthetic page with neutral real-page
memory.

crates/obscura-render/src/lib.rs

@@ -51,6 +51,17 @@ pub mod inline {
         pub fn try_build(&mut self, _tree: &DomTree, _id: NodeId, _styles: &HashMap<NodeId, crate::LayoutStyle>) -> Option<usize> {
             None
         }
+        /// See `try_build`: inline runs likewise fall back to word-split
+        /// geometry in layout-only builds.
+        pub fn try_build_run(
+            &mut self,
+            _tree: &DomTree,
+            _parent: NodeId,
+            _run: &[NodeId],
+            _styles: &HashMap<NodeId, crate::LayoutStyle>,
+        ) -> Option<usize> {
+            None
+        }
     }
 }