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/0055-basic-large-stack-array.sx

@@ -13,7 +13,7 @@
 
 #import "modules/std.sx";
 
-checksum :: () -> s64 {
+checksum :: () -> i64 {
     buf : [65536]u8 = ---;
     i := 0;
     while i < 65536 {
@@ -29,14 +29,14 @@ checksum :: () -> s64 {
     return sum;
 }
 
-big :: () -> s64 {
-    buf : [131072]s64 = ---;
+big :: () -> i64 {
+    buf : [131072]i64 = ---;
     buf[0] = 11;
     buf[131071] = 31;
     return buf[0] + buf[131071];
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     out(int_to_string(checksum()));
     out("\n");
     out(int_to_string(big()));