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/1202-ffi-cc-c-large-aggregate.sx

@@ -14,17 +14,17 @@
 #import "modules/std.sx";
 
 Wide :: struct {
-    a: s64;
-    b: s64;
-    c: s64;
-    d: s64;
+    a: i64;
+    b: i64;
+    c: i64;
+    d: i64;
 }
 
-accept_c :: (w: Wide) -> s64 callconv(.c) {
+accept_c :: (w: Wide) -> i64 callconv(.c) {
     w.a + w.b + w.c + w.d
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     w := Wide.{ a = 1, b = 10, c = 100, d = 1000 };
     if accept_c(w) != 1111 { return 1; }
     0