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/0028-basic-functions.sx

@@ -4,23 +4,23 @@
 #import "modules/std/test.sx";
 pkg :: #import "tests/fixtures/testpkg";
 
-add :: (a: s32, b: s32) -> s32 { a + b }
+add :: (a: i32, b: i32) -> i32 { a + b }
 
-mul :: (a: s32, b: s32) -> s32 { a * b }
+mul :: (a: i32, b: i32) -> i32 { a * b }
 
 identity :: (x: $T) -> T { x }
 
-pair_add :: (a: $T, b: $U) -> s64 {
-    cast(s64) a + cast(s64) b
+pair_add :: (a: $T, b: $U) -> i64 {
+    cast(i64) a + cast(i64) b
 }
 
-typed_sum :: (..args: []s32) -> s32 {
+typed_sum :: (..args: []i32) -> i32 {
     result := 0;
     for args (it) { result = result + it; }
     result
 }
 
-apply :: (f: (s32, s32) -> s32, x: s32, y: s32) -> s32 {
+apply :: (f: (i32, i32) -> i32, x: i32, y: i32) -> i32 {
     f(x, y)
 }
 
@@ -28,11 +28,11 @@ void_return :: () {
     return;
 }
 
-implicit_return :: (x: s32) -> s32 {
+implicit_return :: (x: i32) -> i32 {
     x * 2
 }
 
-early_return :: (x: s32) -> s32 {
+early_return :: (x: i32) -> i32 {
     if x > 10 { return 99; }
     x
 }
@@ -53,7 +53,7 @@ main :: () {
     print("typed-const: {}\n", TYPED_PI);
 
     // Variable with default init
-    di : s32;
+    di : i32;
     print("default-init: {}\n", di);
 
     // Implicit return
@@ -68,7 +68,7 @@ main :: () {
     print("void-return: ok\n");
 
     // Generic — single param
-    print("generic-s32: {}\n", identity(42));
+    print("generic-i32: {}\n", identity(42));
     print("generic-f32: {}\n", identity(1.5));
     print("generic-bool: {}\n", identity(true));
 
@@ -76,7 +76,7 @@ main :: () {
     print("generic-multi: {}\n", pair_add(10, 20));
 
     // Lambda
-    double :: (x: s32) => x * 2;
+    double :: (x: i32) => x * 2;
     print("lambda: {}\n", double(7));
 
     // Lambda with return type
@@ -84,7 +84,7 @@ main :: () {
     print("lambda-ret: {}\n", halve(10.0));
 
     // Local function (non-lambda)
-    local_add :: (a: s32, b: s32) -> s32 { a + b }
+    local_add :: (a: i32, b: i32) -> i32 { a + b }
     print("local-fn: {}\n", local_add(3, 4));
 
     // Nested function calls
@@ -94,11 +94,11 @@ main :: () {
     print("varargs: {}\n", typed_sum(1, 2, 3, 4, 5));
 
     // Spread
-    spread_arr : [3]s32 = .[10, 20, 30];
+    spread_arr : [3]i32 = .[10, 20, 30];
     print("spread: {}\n", typed_sum(..spread_arr));
 
     // Function pointers
-    fp : (s32, s32) -> s32 = add;
+    fp : (i32, i32) -> i32 = add;
     print("fp: {}\n", fp(3, 4));
     fp = mul;
     print("fp-reassign: {}\n", fp(3, 4));