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/0040-basic-block-value.sx

@@ -12,22 +12,22 @@
 #import "modules/std.sx";
 
 // Implicit return: trailing expression, no `;`.
-double :: (n: s32) -> s32 { n * 2 }
+double :: (n: i32) -> i32 { n * 2 }
 
 // if/else as a value — each branch's last expression has no `;`.
-sign :: (n: s32) -> s32 {
+sign :: (n: i32) -> i32 {
     if n < 0 { -1 } else if n > 0 { 1 } else { 0 }
 }
 
 // A value-producing block bound to a name.
-sum3 :: (a: s32, b: s32, c: s32) -> s32 {
+sum3 :: (a: i32, b: i32, c: i32) -> i32 {
     t := { x := a + b; x + c };   // block value is `x + c`
     t
 }
 
 // Match arms keep their `;` (exempt): the arm `;` is an arm terminator, so each
 // arm still yields its expression as the match value.
-classify :: (n: s32) -> s32 {
+classify :: (n: i32) -> i32 {
     if n == {
         case 0:  100;
         case 1:  10;
@@ -35,8 +35,8 @@ classify :: (n: s32) -> s32 {
     }
 }
 
-main :: () -> s32 {
-    total : s32 = 0;
+main :: () -> i32 {
+    total : i32 = 0;
     total = total + double(10);       // 20
     total = total + sign(-7);         // -1
     total = total + sum3(1, 2, 3);    // 6