Add get_charts_in_layout_order(), a depth-first walk of a dashboard's
position_json that returns its charts in visual order including tab-nested
charts. De-duplicates charts placed more than once, skips stale layout entries
with no backing chart, and appends charts absent from the layout at the end
ordered by id. Guards against cyclic layouts. Adds unit tests.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
