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.

selfhost/backend/emit_call.ludic

@@ -249,6 +249,15 @@ function emit_ns_call(ns: pointer, meth: pointer, e: Node) -> Val {
       if g_windowed { emit("  call void @splash_hide()\n") }
       return val("0", "void")
     }
+    if (meth == "set_icon") {
+      # The Dock tile. A bundled app takes it from CFBundleIconFile and this is a
+      # harmless no-op there; a bare `ludic build` binary has no bundle and no icon
+      # at all without it. Headless has no AppKit, so it compiles to nothing.
+      use_pak()
+      let ip = arg_code(e, 0)
+      if g_windowed { emit(`  call void @lp_app_icon(ptr {ip})\n`) }
+      return val("0", "void")
+    }
     perr(`unknown builtin App.{meth}`)
   }
   if (ns == "Pool") {

selfhost/backend/stdlib/emit_pak.ludic

@@ -117,6 +117,7 @@ function emit_pak_prelude() -> void {
   if g_windowed {
     emith("declare void @splash_show(ptr, i32, i32)\n")
     emith("declare void @splash_hide()\n")
+    emith("declare void @app_set_icon(ptr, i32)\n")
   }
 
   emit_pak_mount()
@@ -300,5 +301,19 @@ function emit_pak_boot() -> void {
     emith("show:\n  %len = load i32, ptr %lp\n  %bg = load i32, ptr @L_pak_splashbg\n")
     emith("  call void @splash_show(ptr %blob, i32 %len, i32 %bg)\n  br label %out\n")
     emith("out:\n  ret void\n}\n")
+
+    # App.set_icon(path): the Dock tile for a binary that has no bundle to take one
+    # from. Read 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 the bytes, so the buffer goes straight back.
+    emith("define void @lp_app_icon(ptr %path) {\n")
+    emith("entry:\n  %f = call ptr @lp_pak_open(ptr %path, ptr @.pak_rb)\n  %bad = icmp eq ptr %f, null\n  br i1 %bad, label %out, label %go\n")
+    emith("go:\n  %s1 = call i32 @fseek(ptr %f, i64 0, i32 2)\n  %n64 = call i64 @ftell(ptr %f)\n  %s2 = call i32 @fseek(ptr %f, i64 0, i32 0)\n")
+    emith("  %tiny = icmp sle i64 %n64, 0\n  br i1 %tiny, label %shut, label %read\n")
+    emith("read:\n  %buf = call ptr @malloc(i64 %n64)\n  %rd = call i64 @fread(ptr %buf, i64 1, i64 %n64, ptr %f)\n")
+    emith("  %c1 = call i32 @fclose(ptr %f)\n  %n32 = trunc i64 %rd to i32\n")
+    emith("  call void @app_set_icon(ptr %buf, i32 %n32)\n  call void @free(ptr %buf)\n  ret void\n")
+    emith("shut:\n  %c2 = call i32 @fclose(ptr %f)\n  ret void\n")
+    emith("out:\n  ret void\n}\n")
   }
 }