fix: format castable mixin

[ronald-urbina]

Note

Low Risk
Pure formatting/whitespace changes in castable-mixin.js with no intended runtime behavior changes; low risk aside from inadvertent formatting-related syntax mistakes.

Overview
Applies code-style formatting updates to packages/castable-video/castable-mixin.js (trailing commas, line wrapping, and minor expression reformatting) without changing casting behavior or APIs.

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