lang: introduce cstring — the C-boundary string (Odin model)

cstring is ONE pointer to a null-terminated u8 buffer, C's char*: thin
(8 bytes, no length; cstring_len walks to the terminator), crossing
#foreign boundaries verbatim in both directions, with ?cstring as the
nullable case lowering to the same bare pointer (null = absent).

Conversion discipline mirrors Odin: a string LITERAL coerces implicitly
(its bytes are terminated constants); any other string is rejected with
a diagnostic naming to_cstring (it may be an unterminated view); and
cstring never coerces to string implicitly — from_cstring(c) is the
explicit zero-copy view, pricing the strlen.

Plumbing: TypeId/TypeInfo builtin slot 18 (first_user 19), name
classifiers, size/align/name tables, LLVM ptr lowering, the ?T pointer
niche, the xx pointer ladder, the literal-gated coercion plan
(isConstString + data_ptr), and the reserved-spelling set. std gains
cstring_len/from_cstring/to_cstring (fmt.sx, re-exported); the old
cstring(size) allocator helper is renamed alloc_string everywhere;
getenv migrates to (name: cstring) -> ?cstring as the canonical user
and env() drops its manual strlen/memcpy.

Pinned: examples/1222 (FFI both directions, literal coercion,
?cstring null paths, round trip) and examples/1173 (both coercion
diagnostics); FAIL pre-feature. The alloc_string rename + getenv
signature shift the .ir snapshots — regenerated. zig build test
426/426; run_examples 604/604.

Spec: reserved spelling + cstring section + C-interop rows.

src/backend/llvm/types.zig

@@ -64,13 +64,14 @@ pub const TypeLowering = struct {
             .bool => self.e.cached_i1,
             .error_set => self.e.cached_i32, // u32 tag id on the error channel
             .string => self.e.getStringStructType(),
+            .cstring => self.e.cached_ptr,
             .pointer, .many_pointer, .function => self.e.cached_ptr,
             .closure => self.e.getClosureStructType(),
             .slice => self.e.getStringStructType(), // same {ptr, i64} layout
             .optional => |opt| {
                 // ?*T / ?fn → bare pointer (null = none)
                 const child_info = self.e.ir_mod.types.get(opt.child);
-                if (child_info == .pointer or child_info == .many_pointer or child_info == .function) {
+                if (child_info == .pointer or child_info == .many_pointer or child_info == .function or child_info == .cstring) {
                     return self.e.cached_ptr;
                 }
                 if (child_info == .closure) {