docs(ffi-linkage): document extern/export linkage keywords (Phase 4)

readme.md

@@ -409,6 +409,18 @@ printf :: (fmt: [:0]u8, args: ..Any) -> i32 #foreign libc;
 write_fd :: (fd: i32, buf: [*]u8, count: u64) -> i64 #foreign libc "write";
 ```
 
+`extern` / `export` are the keyword surface for C linkage. `extern` is the modern
+spelling of `#foreign` (import); `export` is its dual — define a function in sx and
+expose it under the C ABI so C can call back in. Both imply `callconv(.c)` and take
+the same optional `[LIB] ["csym"]` rename tail; they also apply to data globals and
+to Obj-C / JNI runtime-class aggregates (postfix after the `#objc_class(…)` directive).
+```sx
+abs       :: (x: i32) -> i32 extern;             // import (== `#foreign`)
+sx_square :: (x: i32) -> i32 export { x * x }    // define + expose to C
+__stdinp  : *void extern;                        // extern data global
+NSObject  :: #objc_class("NSObject") extern { alloc :: () -> *NSObject; }  // reference a runtime class
+```
+
 Direct C header import:
 ```sx
 #import c {