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/0212-generics-array-arg-slice-param.sx

@@ -10,7 +10,7 @@
 
 #import "modules/std.sx";
 
-P :: struct { x: s64; y: s64; }
+P :: struct { x: i64; y: i64; }
 
 first :: (xs: []$T) -> T {
     return xs[0];
@@ -20,8 +20,8 @@ last :: (xs: []$T) -> T {
     return xs[xs.len - 1];
 }
 
-main :: () -> s32 {
-    a : [3]s64 = ---;
+main :: () -> i32 {
+    a : [3]i64 = ---;
     a[0] = 7; a[1] = 8; a[2] = 9;
     print("{}\n", first(a));
     print("{}\n", last(a));
@@ -35,7 +35,7 @@ main :: () -> s32 {
     ps[1] = .{ x = 3, y = 4 };
     print("{}\n", first(ps).y);
 
-    s : []s64 = a;
+    s : []i64 = a;
     print("{}\n", first(s));
     return 0;
 }