fix(cli): preserve snapshot times and clarify occlusion opt-out

[miguel-heygen]

Summary

Repeated snapshot --at flags previously discarded every timestamp except the last one. Repeated flags, comma-separated lists, and mixed forms now combine in order so every requested frame is captured.

text_occluded also suggested the intentional-occlusion opt-out without identifying its target, while the skill reference implied it belonged on the covering element. Both now state that data-layout-allow-occlusion belongs on the covered text or one of its ancestors, never the covering element.

Test plan

• bun run --cwd packages/cli test — 1,750 passed, 2 skipped
• bun run --cwd packages/cli typecheck
• bun run lint
• bun run format:check
• bun run build
• Built CLI: snapshot ... --at 0.2 --at 0.8 --no-end captured both requested PNGs

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