feat: declare the kernel-abi layer this package supplies

mcpp resolves the target side per layer after the dependency graph is known,
and reads which layer a package fills from its capabilities. Declaring
`mcpp:kernel-abi=openkal` states that this package implements the platform
interface a C library sits on, and that it answers to the name `openkal`.

The engine knows the three layer names and none of the implementations that
fill them, so nothing about openkal appears in mcpp for this to work.

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

Author: Sunrisepeak

mcpp.toml

@@ -17,6 +17,16 @@ name        = "openkal-opensbi"
 version     = "0.1.0"
 description = "An implementation of openkal on the RISC-V Supervisor Binary Interface, portable across every machine whose firmware provides one"
 license     = "Apache-2.0"
+
+# The layer this package supplies, in the vocabulary the engine resolves.
+#
+# `mcpp:kernel-abi` names the platform interface a C library sits on. On a
+# traditional stack that seam is unnamed — a C library issues system calls or
+# calls the platform's own entry points directly — and naming it is what lets
+# one C library sit above several platforms. `=openkal` is the interface this
+# package answers to; several packages answer to it and the engine knows none
+# of them by name.
+provides    = ["mcpp:kernel-abi=openkal"]
 authors     = ["mcpplibs"]
 repo        = "https://github.com/mcpplibs/openkal-opensbi"