Add container visualization for accessibility snapshots

[RoyalPineapple]

Adds container visualization to accessibility snapshot legends without changing element overlay behavior.

Summary

• New showContainers configuration flag turns on a hierarchical legend that groups elements by their accessibility container with dashed borders and per-container badges
• Snapshot image and element overlays are baked into a single flat UIImage before the legend enters layout, so overlay positions can never be shifted by legend placement
• Parser/render order swap fixes a pre-existing overlay misalignment for nested UIHostingController content (e.g. SemanticGroupWrapper)
• Demo coverage for every container shape the parser produces (semanticGroup × 4 info variants, list, landmark, dataTable, tabBar) plus a SwiftUI Table reveal
• iOS 26.4 reference images recorded for the new tests

Details

Snapshot + overlay baking

PreParsedAccessibilitySnapshotView.init composites the snapshot image and all element overlays into a single flat UIImage up front. The rendered snapshotWithOverlays body is just Image(uiImage:) — no ZStack of overlays on top of the image. This decouples overlay alignment from the surrounding SwiftUI layout, so the legend (right, below, multi-column, future flow layouts) can never shift overlay coordinates.

Baking uses drawHierarchy(afterScreenUpdates: true) inside a temporary UIWindow with safeAreaRegions = [] so SwiftUI content renders correctly without safe-area offsets.

Overlay alignment fix

drawHierarchy(afterScreenUpdates: true) mutates layout mid-render for views that defer layout until the render cycle — notably nested UIHostingController instances inside UIViewRepresentable wrappers (e.g. SemanticGroupWrapper). When parsing happened after rendering, the parser read accessibilityFrame values from a post-render layout that no longer matched what the image captured, shifting the first group's overlay.

Fix: swap the parse/render order in AccessibilitySnapshotBaseView.parseAccessibility() so marker frames and the rendered image both come from the same pre-render layout state.

Container legend

When showContainers is enabled, the legend renders a hierarchical view that groups elements by their accessibility containers using dashed borders and container badges. Container entries expand to the full legend column width so multi-word badges like "Data Table (3 × 4)" stay on one line. The snapshot area is identical regardless of showContainers — only the legend content changes.

Supported container types in the badge label:

• semanticGroup(label, value, identifier) → "label: value", or identifier, or "Semantic Group"
• list → "List"
• landmark → "Landmark"
• dataTable(rows, cols) → "Data Table (r × c)"
• tabBar → "Tab Bar"

New types:

• HierarchyColorAssignment — assigns color indices to hierarchy nodes (elements use traversal-order position matching overlay indices; containers use a separate counter)
• ContainerLegendEntryView / ContainerBadge — dashed-border legend entry with layered-icon badge
• HierarchyLegendView — recursive hierarchical legend renderer

Demos and tests

Each container type now has a focused demo + test under AccessibilitySnapshotPreviewsDemo:

• ContainerDemo — four semanticGroup info variants (label only, label + value, identifier only, all three)
• ListContainerDemo — accessibilityContainerType = .list
• LandmarkContainerDemo — accessibilityContainerType = .landmark
• TabBarContainerDemo — accessibilityTraits = .tabBar on a non-UITabBar view
• DataTableContainerDemo — accessibilityContainerType = .dataTable with real UIAccessibilityContainerDataTableCell elements reporting per-cell row/column ranges, so the legend shows "Data Table (3 × 4)" plus each cell's row/column context
• SwiftUITableDemo — exposes how SwiftUI's native Table collapses to a one-column List on iPhone; the parser still emits a .dataTable container but with rowCount = 0, columnCount = 0 and no UIAccessibilityContainerDataTableCell elements

Shared AccessibilityContainers.swift provides:

• A generic UIViewRepresentable that wraps SwiftUI content in a UIView reporting a configured UIAccessibilityContainerType or the .tabBar trait
• An AccessibilityDataTable UIViewRepresentable that renders a 2D grid of labels and exposes each cell as a real UIAccessibilityElement conforming to UIAccessibilityContainerDataTableCell

Other changes

• FBSnapshotTestCase+SwiftUI: sets hostingController.safeAreaRegions = [] on iOS 16.4+ in the config-based overload, preventing drawHierarchy from baking status bar / safe-area offsets into the snapshot
• AccessibilitySnapshotConfiguration: new showContainers: Bool = false flag
• ParsedAccessibilityData: now also carries the full hierarchy: [AccessibilityHierarchy] tree (not just flattened markers) so the container legend can render

Test plan

• All existing snapshot tests pass
• Six new container-demo tests produce overlays aligned 1:1 with snapshot content
• Container legend correctly groups elements with dashed borders and full-width badges
• DataTable demo reports per-cell row/column context (no NSNotFound noise)
• iOS 26.4 reference images recorded for all new tests

🤖 Generated with Claude Code

[RoyalPineapple]

Still in progress