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/0512-packs-pack-runtime-index.sx

@@ -11,9 +11,9 @@
 
 #import "modules/std.sx";
 
-count_anys :: (..$args) -> s64 {
-    total : s64 = 0;
-    i : s64 = 0;
+count_anys :: (..$args) -> i64 {
+    total : i64 = 0;
+    i : i64 = 0;
     while i < args.len {
         x : Any = args[i];   // ERROR: runtime index into a comptime-only pack
         _ = x;
@@ -23,7 +23,7 @@ count_anys :: (..$args) -> s64 {
     return total;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("{}\n", count_anys(10, "hi", 2.5, true));
     return 0;
 }