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/1216-ffi-08-foreign-in-method.sx

@@ -18,8 +18,8 @@
 
 // ── 1. Struct method calling a #foreign fn ───────────────────────────
 Counter :: struct {
-    seed: s32 = 0;
-    next :: (self: *Counter) -> s32 {
+    seed: i32 = 0;
+    next :: (self: *Counter) -> i32 {
         v := ffi_method_helper(self.seed);
         self.seed += 1;
         v
@@ -28,21 +28,21 @@ Counter :: struct {
 
 // ── 2. Protocol impl method calling a #foreign fn ────────────────────
 Doubler :: protocol {
-    doubled :: (self: *Self) -> s32;
+    doubled :: (self: *Self) -> i32;
 }
 
 impl Doubler for Counter {
-    doubled :: (self: *Counter) -> s32 {
+    doubled :: (self: *Counter) -> i32 {
         ffi_method_helper(self.seed) * 2
     }
 }
 
 // ── 3. Closure body calling a #foreign fn ────────────────────────────
-make_adder :: (bias: s32) -> Closure(s32) -> s32 {
-    closure((x: s32) -> s32 => ffi_method_helper(x) + bias)
+make_adder :: (bias: i32) -> Closure(i32) -> i32 {
+    closure((x: i32) -> i32 => ffi_method_helper(x) + bias)
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     c : Counter = .{ seed = 1 };
 
     // 1. struct method