lang: rename signed integer types sN -> iN

Surface rename of the signed integer family: s1..s64 become i1..i64
(u1..u64, usize, isize unchanged). 'string' keeps the s-prefix arm in
name classification; width parsing moves to the i-prefix arm next to
isize.

Internal TypeId tags follow the surface (.s8/.s16/.s32/.s64 ->
.i8/.i16/.i32/.i64), as do mono-key mangle fragments (ptr_i64,
tu_i64_bool) and all display/diagnostic formatting (i{d}).

Migrated in the same sweep: stdlib + examples + issue repros + FFI C
companions (shared symbol names like ffi_id_i64), expected
stdout/stderr/ir snapshots, specs.md, readme.md, CLAUDE.md/AGENTS.md,
implementation_plan.md, docs/, issue writeups. Vendored stb_image and
historical flow state left untouched.

zig build test: 426/426; examples suite: 595/595.

examples/0802-memory-pointers.sx

@@ -21,8 +21,8 @@ main :: () {
     np : *Vec2 = null;
 
     // many-pointer indexing
-    arr : [5]s32 = .[10, 20, 30, 40, 50];
-    mp : [*]s32 = @arr[0];
+    arr : [5]i32 = .[10, 20, 30, 40, 50];
+    mp : [*]i32 = @arr[0];
     print("mp[0] = {}\n", mp[0]);
     print("mp[2] = {}\n", mp[2]);
 }