Give the Dock an icon outside the bundle, and macOS 26 the Icon Composer one

[huntharo]

This fixes two problems with disktree's icon on macOS.

• A run outside the app bundle has no icon. make run, cargo run and the bare release binary have no bundle for macOS to read, so the Dock shows a generic "exec" tile.
• macOS 26 has to guess at the bundled icon. The bundle ships only a legacy .icns, so macOS 26 squeezes it into its own icon shape instead of drawing a Liquid Glass icon. On 26.6.1 the mark's square panel runs into the corners of the mask, as the before image shows. On 26.6.2 a padded .icns like this one comes out as a dark tile on a light grey plate. PwrSnap and PwrGit hit that and wrote it up in fix(desktop): ship an Icon Composer .icon alongside the legacy .icns pwrdrvr/PwrSnap#563.

The fix follows what PwrSnap and PwrAgent settled on.

Before and after

In the Dock, left to right: an unbundled run before this PR, main's bundle, an unbundled run of this branch, and this branch's bundle.

The bundled icon as macOS 26.6.1 draws it, read back through NSWorkspace.shared.icon(forFile:) at 512 px:

What changes

• An Icon Composer icon in the bundle. packaging/macos/AppIcon.icon is the package: icon.json with the mark's background as the fill, and one layer drawn by sips from the new AppGlyph.svg, the mark without its background. When xcrun actool is 26 or newer, cargo xtask bundle compiles it into Contents/Resources/Assets.car and adds CFBundleIconName to the plist. AppIcon.icns stays as CFBundleIconFile for macOS 15 and earlier. Without actool 26, as with the Command Line Tools alone, an older Xcode, or the macos-15 runner, the bundle is built as before and the task prints a note.
• A Dock icon for unbundled runs. crates/disktree-app/src/dock_icon.rs checks whether the main bundle names an icon. When it does not, it sets NSApplication.applicationIconImage from a PNG embedded in the binary: DockIcon-glass.png on macOS 26 and later, DockIcon.png before that. It reads the Darwin kernel release to decide, because macOS 26 reports itself as 16.0 to a binary linked against an older SDK. AppKit comes through the objc2 crates GPUI already builds, with one unsafe call, allowed with its reason the way macos.rs does.
• Rendering the glass. macOS never applies Liquid Glass to an image set at run time, so the glass is baked into DockIcon-glass.png. make dock-icons compiles the package into a throwaway bundle and reads back what macOS draws for it. It then checks that the tile lands where the flat icon's does and that the Mac is set to the Default icon style. It needs macOS 26 and Xcode 26. Run it after either SVG changes.
• CI. A new Bundle (macOS 26) job builds the bundle on macos-26 and checks the catalog and both plist keys are there. The macos-15 job keeps covering the bundle without it. The aarch64 release build moves to macos-26 so the shipped app carries the catalog. There is no macOS 26 Intel runner, so the x86_64 build keeps the legacy icon alone.

The mark's size (last commit, droppable)

At the full canvas, the mark's square panel reaches into the mask's corners and gets clipped, which is what main's bundle shows above. Apple's documentation points to the grid in its Design Resources templates rather than giving a number. So I rendered sizes through macOS beside the icons around disktree in the Dock. The last commit draws the panel at 700 pt of the 1024 pt canvas, 162 pt in from each edge, with rounded corners, so it reads as a squircle inside the squircle. That matches the weight of Slack's glyph, 1Password's ring, and the PwrSnap and PwrGit marks.

The size change is a commit of its own so it can be dropped. Without it, the mark fills the canvas:

Verified

• cargo xtask lint and cargo xtask test pass on macOS 26.6.1 with Xcode 26.5. CI passes all seven checks, including the new macOS 26 bundle job.
• cargo xtask bundle here gives Assets.car and AppIcon.icns in Resources, CFBundleIconName = Icon, CFBundleIconFile = AppIcon, and a valid signature.
• The Dock images are real captures of running apps, not the PNGs.
• The "main's bundle" images come from this branch's bundle with Assets.car and CFBundleIconName removed and the bundle re-signed. AppIcon.svg and the icns it is drawn into are unchanged from main, so that is the icon main ships.
• Not verified: macOS 26.6.2, where the plate shows, since this Mac runs 26.6.1. Also not verified on macOS 15, which should still draw AppIcon.icns as before.