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/1602-platform-http-server.sx

@@ -7,7 +7,7 @@
 
 Logger :: struct {
     prefix: string;
-    count: s64;
+    count: i64;
 }
 
 log :: (logger: *Logger, msg: string) {
@@ -17,7 +17,7 @@ log :: (logger: *Logger, msg: string) {
 
 
 
-main :: () -> s32 {
+main :: () -> i32 {
     PORT :: 8080;
 
     fd := socket(AF_INET, SOCK_STREAM, 0);
@@ -26,7 +26,7 @@ main :: () -> s32 {
         return 1;
     }
 
-    opt : s32 = 1;
+    opt : i32 = 1;
     setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, @opt, 4);
 
     addr := SockAddr.{ sin_len = 16, sin_family = 2, sin_port = htons(PORT) };
@@ -63,7 +63,7 @@ main :: () -> s32 {
     0
 }
 
-handle :: (client: s32) {
+handle :: (client: i32) {
     // Read request
     buf : [4096]u8 = ---;
     read(client, buf, buf.len);