docs: abi is the kernel↔runtime contract, not spoken by applications

Correct the abi.zig header (and the matching build.zig / README lines): the syscall ABI
is the private contract between the kernel and the runtime library, not something every
user program speaks. danos applications call the `runtime` (the stable, danos-native
ABI); the runtime is the only thing that issues system calls, and POSIX layers over the
runtime — the same split as libSystem on macOS or win32 over the NT syscalls. The call
numbers here are an implementation detail the runtime hides and may renumber, not a
public interface.

build.zig

@@ -102,7 +102,7 @@ pub fn build(b: *std.Build) void {
     // declares which one it speaks (no target is set, so each inherits the target of
     // whichever binary imports it). See docs/coding-standards.md.
     //   boot-handoff : loader <-> kernel (BootInformation, framebuffer, VM layout)
-    //   abi          : kernel <-> user, core (SystemCall, mmap prot flags, page_size)
+    //   abi          : kernel <-> runtime, core (SystemCall, mmap prot flags, page_size)
     //   device-abi   : kernel <-> user, devices (DeviceDescriptor, DeviceClass, ...)
     const boot_handoff_module = b.addModule("boot-handoff", .{
         .root_source_file = b.path("system/boot-handoff.zig"),