v15.6: skip invisible text, charpos↔index mapping, completions overlay
Major architectural change: ns_ax_buffer_text now skips text with the 'invisible' property using TEXT_PROP_MEANS_INVISIBLE. Accessibility text matches what the user sees on screen. New ns_ax_visible_run struct tracks charpos↔ax-index mapping for each visible text run. All 8 index conversion sites updated. Fixes wrong line reading in dired, completions, and any buffer with invisible text. Completions announcement now detects completions-highlight overlay and reads the full highlighted candidate text instead of partial line. 2000-line patch, 54 references to new mapping infrastructure.
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