fix(ldf): treat guards on unseen macros as undecidable, and honor #if 0 hints

crates/fbuild-header-scan/src/lib.rs

@@ -9,9 +9,13 @@
 mod scanner;
 mod walker;
 
-pub use scanner::{IncludeKind, IncludeRef, Span, active_defines, scan, scan_active};
+pub use scanner::{
+    IncludeKind, IncludeRef, Span, active_defines, defined_macro_names, scan, scan_active,
+    scan_active_with_known,
+};
 pub use walker::{
-    WalkResult, WalkState, walk, walk_active, walk_with_state, walk_with_state_active,
+    WalkResult, WalkState, collect_defined_macro_names, walk, walk_active, walk_with_state,
+    walk_with_state_active, walk_with_state_active_known,
 };
 
 /// Bumped whenever the scanner output shape changes. Mixed into cache keys so a