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>

tests/run/abi.h

@@ -0,0 +1,25 @@
+#include <stdint.h>
+typedef struct { float x, y; } Vec2;                      // SSE: <2 x float>
+typedef struct { float x, y, z; } Vec3;                   // SSE, SSE (float tail)
+typedef struct { uint8_t r, g, b, a; } Color;             // INTEGER: i32
+typedef struct { double d; int i; } Mixed;                // SSE + INTEGER
+typedef struct { int64_t a, b; } Pair;                    // INTEGER, INTEGER
+typedef struct { int64_t a, b, c; } Big;                  // MEMORY (24 bytes)
+typedef struct { char name[8]; int count; } Named;        // fixed array inside
+typedef struct { Vec2 pos; Color color; } Sprite;         // nested
+typedef struct Item { int value; struct Item *next; } Item;
+
+Vec2 vec2_add(Vec2 a, Vec2 b);
+Vec3 vec3_scale(Vec3 v, float k);
+Color color_invert(Color c);
+Mixed mixed_bump(Mixed m);
+Pair pair_swap(Pair p);
+Big big_sum(Big a, Big b);
+int64_t big_total(Big b);
+int named_len(Named n);
+Sprite sprite_move(Sprite s, float dx);
+int item_sum(Item *it);
+// many structs: runs out of registers, later ones go on the stack
+double many(Vec2 a, Vec2 b, Vec2 c, Vec2 d, Vec2 e, Vec2 f, Vec2 g, Vec2 h, Vec2 i, Pair p1, Pair p2, Pair p3, Pair p4);
+int64_t apply_pair(Pair (*fn)(Pair, int64_t), Pair p, int64_t k);
+float apply_vec(Vec2 (*fn)(Vec2), Vec2 v);