restricted c header imports

LANGUAGE.md

@@ -32,6 +32,9 @@
 - Apple Silicon C ABI scalar and pointer lowering, including narrow integer extension attributes
 - directory packages with merged declarations
 - file-local relative imports, aliases, and qualified member access
+- relative `.h` imports as synthetic package namespaces
+- transitive external C function prototypes, typedef chains, C scalars, and pointers to opaque C records
+- reference-time diagnostics for unsupported imported C declarations
 
 ### compiler behavior
 
@@ -39,6 +42,8 @@
 - lazy semantic checking of demanded function specializations
 - demand-driven LLVM declarations for referenced foreign functions
 - ordered linking of additional c sources, objects, and libraries
+- replaceable dynamically loaded libclang C-import backend
+- per-compilation C-header import caching by canonical path, target, ordered include paths, and ordered defines
 - static, eager runtime, and deferred problematic globals
 
 ## PLANNED
@@ -53,9 +58,7 @@
 - tuples and native variadic functions
 - C unions, C enums, and by-value C record ABI lowering
 
-### c imports
+### advanced c imports
 
-- c headers imported as synthetic package namespaces
-- typedefs, enums, opaque records, and external variables
-- function pointers, callbacks, macros, and static inline functions
+- C enums, external variables, function pointers, callbacks, macros, and static inline functions
 - target-specific by-value C record and union ABI lowering