Structs, arrays, owned strings, memory model, modules, debug info

- struct with default values; constructors Point(1, 2) / Point(x: 1); field access, auto-deref through pointers
- []T arrays with bounds checks, literals, for-each, push/pop/insert/remove/clear/resize/find/contains/join/slice/reverse/sort
- str carries its length: .len, s[i], +, comparison, sub/find/split/trim/upper/lower/replace/repeat/bytes, str(x), int(s), float(s)
- Memory: values own their memory and are freed at the end of their block; assignment copies, temporaries move;
  parameters are lent and only copied when the function changes them; defer; new()/free() for manual heap memory
- input(), read_file/write_file/file_exists, exit(), fn main([]str args)
- Finch modules: import name, module.fn(), module.Struct
- C structs from headers (fields, fixed arrays, nested); passed and returned by value with the System V x86-64 ABI;
  addr(fn) gives C a callback
- link "file.c" compiles and links your own C code
- -g emits DWARF debug info (gdb/lldb)
- Small C runtime (runtime/finch_rt.c) embedded in the compiler, cached in ~/.cache/finch
- Tests: 58, all clean under valgrind (MEMCHECK=1 tests/run.sh)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/cimport.h

@@ -2,11 +2,13 @@
 
 #include "ast.h"
 
+#include <map>
 #include <string>
 #include <unordered_map>
 #include <vector>
 
 // What Finch learned from `import "something.h"`.
+// C structs by value appear in types as Named{name, module "C"}.
 
 struct CFunc {
     std::string name;
@@ -27,11 +29,28 @@ struct CGlobal {  // extern variables like stdout
     Type type;
 };
 
+struct CField {
+    std::string name;
+    Type type;
+    long long offset = 0;  // bytes
+    long long size = 0, align = 1;
+    bool hidden = false;   // unions, bitfields, unmappable types: kept for layout only
+};
+
+struct CStruct {
+    std::string name;
+    std::vector<CField> fields;
+    long long size = 0, align = 1;
+    std::string unsupported;  // why it can't be used by value (pointers to it still work)
+    bool hasHidden = false;
+};
+
 struct CImports {
     std::unordered_map<std::string, CFunc> fns;
     std::unordered_map<std::string, CConst> consts;
     std::unordered_map<std::string, CGlobal> globals;
+    std::map<std::string, CStruct> structs;
 };
 
-// `dir` is the folder of the .fn file, searched first for "local.h" headers.
-CImports importHeaders(const std::vector<Import> &imports, const std::string &dir);
+// `dirs`: folders searched first for "local.h" headers (the .fn files' folders).
+CImports importHeaders(const std::vector<Import> &imports, const std::vector<std::string> &dirs);