feat(app): App.set_icon, so a game without a bundle still has an icon

`ludic bundle` builds an AppIcon.icns and macOS reads it out of the .app, so a
shipped game has an icon. `ludic build` produces a bare executable: no bundle, no
CFBundleIconFile, and so no icon at all - macOS draws the generic green "exec"
tile. That is the build a developer runs every day, which is why "the game has no
icon" can be true for months while the bundle is perfect. It was here: the .app's
icns validated against iconutil, the plist was right, the signature was right,
and NSWorkspace rendered the artwork - and the binary beside it still had the
exec tile.

App.set_icon(path) takes the bytes through lp_pak_open, so a packed path and a
loose one both work and this does not repeat Audio.load's trick of taking a
filesystem path only. NSData copies them, so the buffer goes straight back. A
missing or undecodable image leaves the existing icon alone rather than clearing
it; a bundled app is unaffected; headless links no AppKit and compiles it away.

Verified the Cocoa sequence against an ObjC twin doing the same message sends:
before, a bare binary's applicationIconImage is the generic 128x128 tile; after,
it is the 1024x1024 artwork.

Backend change, so selfhost/ludicc.seed.ll is reseeded: the bootstrap fixpoint
and the C-free rebuild from the seed both pass.

docs/language/app/app-set_icon.md

@@ -0,0 +1,31 @@
+---
+id: app-set_icon
+name: App.set_icon
+category: app
+kind: namespace-method
+tokens: App.set_icon
+sig: App.set_icon(path) -> void
+tip: Set the Dock icon from an image the game ships.
+order: 2
+ns: App
+member: set_icon
+---
+
+Sets the application's icon — the Dock tile, the Cmd-Tab switcher, the About box — from an image the game ships.
+
+A **bundled** app already has one: `ludic bundle` builds `AppIcon.icns` from the manifest's `app icon` and macOS reads it from the bundle, so calling this there is harmless and changes nothing you can see. A **plain binary** is the case this exists for. `ludic build` produces an executable with no bundle around it, so macOS has nowhere to find an icon and draws the generic one — which is what a developer looks at every time they run the game.
+
+The path is resolved the same way every other asset is, through the pack first and the filesystem second, so the same call works in a packed build and in a project directory. An image that cannot be found or decoded leaves the current icon alone rather than clearing it.
+
+Parameters:
+- `path` — an image the game ships (PNG, JPEG or TIFF); 1024x1024 is the size macOS wants
+
+```ludic
+program Titled {
+  entry {
+    App.set_icon("assets/app/icon.png")
+  }
+}
+```
+
+Headless builds have no AppKit, so this compiles to nothing there.