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.

library/modules/std.sx

@@ -69,7 +69,10 @@ optional_to_string :: fmt.optional_to_string;
 concat      :: fmt.concat;
 substr      :: fmt.substr;
 path_join   :: fmt.path_join;
-cstring     :: fmt.cstring;
+alloc_string :: fmt.alloc_string;
+cstring_len  :: fmt.cstring_len;
+from_cstring :: fmt.from_cstring;
+to_cstring   :: fmt.to_cstring;
 alloc_slice :: fmt.alloc_slice;
 
 // fmt internals, re-exported only because they were always part of the