macOS: Liquid Glass app icon (macOS 26+) with flattened fallback

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Adds a native Liquid Glass app icon for macOS 26 (Tahoe) and later, built from the repo's own branding: the BS burst logo as a glass layer over the SSB64 wavy-blue backdrop, packaged as an Icon Composer document at assets/BattleShipIcon.icon (whitelisted through the assets/* gitignore like the other branding files).

How it works

package-macos.sh compiles the Icon Composer package with actool when the local Xcode supports the format (Xcode 26+):

• Assets.car ships in Resources and provides the Liquid Glass icon on macOS 26+ via a new CFBundleIconName key (inserted only when the catalog actually shipped, avoiding the generic-icon trap).
• actool also emits a flattened render of the same artwork, which replaces the hand-made AppIcon.icns so pre-Tahoe systems show matching art.
• On older Xcode the block quietly no-ops — flat icns only, no new plist key, byte-identical bundle to today. Nothing about your release environment needs to change for this to be safe.
• US only for now: the JP bundle keeps its distinct flat icon. Happy to do a JP variant of the .icon package if you'd like matching treatment there — or if you'd prefer different layer choices for this one, the source is easy to rework.

Verification

Packaged locally on Apple Silicon (Xcode 26.5) via SSB64_VERSION=us JOBS=4 ./scripts/package-macos.sh: bundle contains Assets.car + both icon keys, and rendering the .app's icon through NSWorkspace (the same pipeline Finder/Dock use) shows the Liquid Glass treatment, with the flattened fallback matching.

🤖 Generated with Claude Code

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One clarifying note: side by side with the current icon you'll notice this looks almost identical, and that is intentional. The main practical difference is rather than shipping a fixed bitmap, the Icon Composer version lets macOS adapt the icon automatically based on Light/Dark and tinted appearance modes, plus whatever styling future OS versions apply. With that said, it's your branding and I want to respect that! So if you'd rather keep the current icon, or want any adjustments, I completely understand. 🙂👍🏼