Refine COVID-19 animation easing and trailing behavior

[jochotecoa]

Motivation

• Improve the visual pacing of the COVID-19 top-10 cases bar chart to reduce abrupt jumps between frames.
• Provide a lighter trailing context so viewers can perceive recent motion without cluttering the plot.
• Use gentler easing to make rank changes feel more natural.

Description

• Adjusted transition_states(year_week, transition_length = 4, state_length = 2, wrap = FALSE) to rebalance transition and hold timing.
• Replaced enter_fade()/exit_fade() with enter_grow() and exit_shrink() to give items a more physical appearance when entering and leaving.
• Swapped shadow_mark() for shadow_trail(max_frames = 12, alpha = 0.15, size = 0.3) to render a soft trailing path, and changed easing to ease_aes("sine-in-out") for smoother motion.
• Kept existing animate() render calls and output targets unchanged.

Testing

• Ran Rscript -e "library(gganimate); library(ggplot2)" to verify required R packages, which failed because Rscript is not available in the environment.
• No animation rendering tests were executed due to the missing R runtime, so visual verification remains pending.

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Codex Task