fixup! C64-Secure ABI demo.

Rename TODO -> NB as discussed in comments.

morello-c64-secure/c64-secure.c

@@ -434,7 +434,8 @@ bool maybe_print_abi_desc(enum ABI from, enum ABI to)
 // Note: Since this is implemented in C, it uses AAPCS64-cap, which we
 // respresent as 'Z'. `can_call()` says that the example ABIs can't call 'Z',
 // but in practice they'll all work well enough to call this helper.
-// TODO: This might fail if the user requests a large number of locals.
+//
+// NB: This might fail if the user requests a very large number of locals.
 struct Next what_next(uintptr_t cfp, uintptr_t csp, size_t pc)
 {
     clear();

morello-c64-secure/impls.s

@@ -128,15 +128,15 @@ C:
         scvalue cfp, cfp, x16
 
         // Derive a new csp from the new cfp.
-        // TODO: For now, we just allow 16KB per function, but we could pick the
+        // NB: For now, we just allow 16KB per function, but we could pick the
         // longest length that guarantees 16-byte alignment (E=4).
         mov x16, #(16 * 1024)
         sub x17, sp, x16
         scvalue c17, cfp, x17
         scbndse c17, c17, x16
         sub x16, sp, w0, uxtw #4    // Allocate locals.
         scvalue csp, c17, x16
-        // TODO: The very last section of stack will be unusable because
+        // NB: The very last section of stack will be unusable because
         // scbnds{e} fails if the required bounds exceed the bounds of <Cn>.
 
 1:      mov c0, cfp