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

examples/hello/mcpp.toml

@@ -13,5 +13,5 @@ runner  = ["qemu-system-riscv64", "-machine", "virt", "-nographic",
            "-no-reboot", "-bios", "default", "-kernel"]
 
 [dependencies]
-openkal         = { git = "https://github.com/mcpplibs/openkal", branch = "feat/openkal-closure" }
+openkal = "0.6.0"
 openkal-opensbi = { path = "../.." }

mcpp.toml

@@ -14,7 +14,7 @@
 [package]
 namespace   = "mcpplibs"
 name        = "openkal-opensbi"
-version     = "0.1.1"
+version     = "0.1.2"
 description = "An implementation of openkal on the RISC-V Supervisor Binary Interface, portable across every machine whose firmware provides one"
 license     = "Apache-2.0"
 
@@ -33,7 +33,7 @@ repo        = "https://github.com/mcpplibs/openkal-opensbi"
 # The contract, not an implementation of it. Declaring it turns a version
 # mismatch into a resolution-time message rather than a link-time one.
 [dependencies]
-openkal = { git = "https://github.com/mcpplibs/openkal", branch = "feat/openkal-closure" }
+openkal = "0.6.0"
 
 # ⭐ WHAT RECEIVES CONTROL, WHICH IS A STATEMENT ABOUT THE PROGRAM.
 #