[xiaolai]

What

On macOS, headings and tables currently expose almost no structure:

• Role::Heading maps to the role string "Heading", which is not a standard AX role, and the node's level is never read.
• Tables, grids and tree grids get the right roles, but no AXRows, AXRowCount, AXColumnCount, AXIndex, cell ranges or column headers (issue Computed table information #10 covers the related computed-table case).

This PR:

• maps Role::Heading to AXHeading (NSAccessibilityHeadingRole, macOS 26), and exposes the level as AXHeadingLevel (1-based) and as AXValue, as WebKit and Chromium do for releases before macOS 26;
• for Table, Grid and TreeGrid: AXRows (row children plus the rows inside RowGroup children), AXRowCount, AXColumnCount and AXColumnHeaderUIElements (the ColumnHeader cells of its rows);
• for rows: AXIndex; for cells (Cell, GridCell, ColumnHeader, RowHeader): AXRowIndexRange and AXColumnIndexRange.

Counts and indices come only from the provider (row_count, column_count, row_index, column_index, spans). When the provider gives none, AXRowCount falls back to the number of rows exposed, and every other structural attribute is withheld through isAccessibilitySelectorAllowed: rather than reported as 0.

AXColumns is not implemented (AppKit marks it optional), and neither are per-cell header links, since AccessKit has no property for them.

accessibilityAttributeValue: now returns NSObject instead of NSString, so that it can return an NSNumber for AXHeadingLevel; the existing string attributes are unchanged.

Testing

On macOS 27 (Apple silicon), with a winit host exposing a book chapter with four headings (levels 1, 2, 1, 2) and a 3 × 2 table with a header row, queried through AXUIElement:

• before: role Heading, role description "Heading", no level; table with no AXRows or counts;
• after: role AXHeading, description "heading", AXHeadingLevel 1, 2, 1, 2; AXRows holding exactly the 3 rows, AXRowCount 3, AXColumnCount 2, each row's AXIndex, each cell's row and column range, and AXColumnHeaderUIElements equal to the two header cells;

cargo fmt --check, cargo clippy -p accesskit_macos --all-targets -- -D warnings and cargo test -p accesskit_macos pass. The adapter has no unit tests to extend; I have not built with Rust 1.87 locally, and nothing here uses newer language or library features. VoiceOver itself has not yet been run against this build.