refactor(deps): resolve openkal through the index rather than a branch

A released package that names a development branch is fragile: the branch is
not part of any release, and deleting it breaks a tarball that has already been
published. openkal 0.6.0 is in the index, so this reference names it.

Claude-Session: https://claude.ai/code/session_01Q4ucduLuSsETHYJ1RkGVVD

Author: Sunrisepeak

mcpp.toml

@@ -1,7 +1,7 @@
 [package]
 namespace   = "mcpplibs"
 name        = "openkal-linux"
-version     = "0.5.2"
+version     = "0.5.3"
 description = "The reference implementation of openkal for Linux, written on the kernel's own system-call interface so that it can be placed beneath a C library as well as above one."
 license     = "Apache-2.0"
 
@@ -18,7 +18,7 @@ authors     = ["mcpplibs"]
 repo        = "https://github.com/mcpplibs/openkal-linux"
 
 [dependencies]
-openkal = { git = "https://github.com/mcpplibs/openkal", branch = "feat/openkal-closure" }
+openkal = "0.6.0"
 
 # The package contributes definitions and no modules. The interface it
 # implements is declared by the specification package, which this package