Keep the arch layer's prose consumer-agnostic; note the x86-64 binding

The generic address-space layer no longer names win32 in its comments: the
khalf_score and gva_translate doc-comments described themselves in terms of
their current Windows consumer, a downward coupling from the stable layer to a
specific, swappable one. Reworded to describe what each primitive does, not who
calls it. Also drop a dangling reference to the renamed engine handle.

State the contract's real scope in memmodel.h: OS-agnostic but architecture-
bound. The address-space key is the x86-64 CR3 (the PML4 base), shared by any
guest OS on x86-64 - CR3 is an ISA register, not a Windows concept; only its
per-process storage (DirectoryTableBase) is win32-specific and stays in the
win32 engine.

src/engine/gva.c

@@ -46,8 +46,8 @@ static int gva_gpa(vmie_mem* m, uintptr_t cr3, uintptr_t va,
 }
 
 /* cold extern translate: GPA of `va` under `cr3`, or -1. Wraps the hot static
- * gva_gpa for cold callers outside this TU (win32 bring-up) without exposing the
- * inlinable hot primitive. Declared in engine-arch.h. */
+ * gva_gpa for cold callers outside this TU without exposing the inlinable hot
+ * primitive. Declared in engine-arch.h. */
 __attribute__((cold))
 int gva_translate(vmie_mem* m, uintptr_t cr3, uintptr_t va, uintptr_t* gpa) {
     return gva_gpa(m, cr3, va, gpa, NULL);