Keep timeline lightbox selection stable as images change

[brsbl]

Human comments

What was wrong

The gallery kept a stale image list and used rendered order to identify repeated images. Timeline updates could lose selection, expose clipped images, or leave navigation unavailable. Navigation wrapped at the ends, and disabled arrow taps dismissed the overlay.

What changed

• Refresh navigation from the loaded, expanded timeline while retaining only the currently displayed image when it becomes ineligible.
• Preserve image occurrence identity through streaming and Markdown normalization using source positions.
• Stop at each end and absorb disabled-arrow taps without dismissing the lightbox.

How you verified

• Remote CI passed at ab809c73d7c0292d94a36c1d44bedb86271a6bfc, including ten gallery regression cases.
• Duplicate inline/footnote regressions failed before the fix and passed afterward, including normalized source offsets and cached streaming blocks.
• Chrome for Testing 153.0.8010.36 confirmed endpoint touch, mouse, and keyboard behavior at 0bd3f0ce1.

Final-candidate browser verification and refreshed screenshots remain unverified: the isolated dev-app route timed out twice. The user requested merging after this gap was reported. Earlier retention, expansion/collapse, and endpoint evidence does not establish final-candidate live streaming behavior.

Historical before and after

Same synthetic timeline, last image, disabled Next tap, and 390 × 844 CSS viewport at 2×. Before: a20b3748c. After: 0bd3f0ce1. These captures predate main integration and are not an exact parent/head comparison for this layer.

Before: tapping disabled Next dismisses	After: tapping disabled Next keeps image 7 / 7 open

BB-Thread-ID: thr_rmv43jkeac

AGENT GENERATED