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/0024-basic-for-list.sx

@@ -4,23 +4,23 @@
 #import "modules/std.sx";
 
 Box :: struct {
-    v: s64;
-    boxed :: (self: Box) -> s64 { self.v }   // value receiver
+    v: i64;
+    boxed :: (self: Box) -> i64 { self.v }   // value receiver
 }
 
-sum_ptr :: (xs: *List(s64)) -> s64 {
-    total : s64 = 0;
+sum_ptr :: (xs: *List(i64)) -> i64 {
+    total : i64 = 0;
     for xs (n) { total = total + n; }         // iterate through a *List
     total
 }
 
-main :: () -> s32 {
-    xs := List(s64).{};
+main :: () -> i32 {
+    xs := List(i64).{};
     xs.append(10);
     xs.append(20);
     xs.append(30);
 
-    s : s64 = 0;
+    s : i64 = 0;
     for xs (n) { s = s + n; }                 // value capture
     print("sum {}\n", s);                      // 60
 
@@ -33,7 +33,7 @@ main :: () -> s32 {
 
     bs := List(Box).{};
     bs.append(.{ v = 7 });
-    bt : s64 = 0;
+    bt : i64 = 0;
     for bs (*b) { bt = bt + b.boxed(); }      // *Box receiver, value-self method
     print("boxes {}\n", bt);                   // 7
     0