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.

library/modules/std/trace.sx

@@ -28,16 +28,16 @@ libc :: #library "c";
 // library/vendors/sx_trace_runtime/sx_trace.c.
 TraceFrame :: struct {
     file: string;
-    line: s32;
-    col: s32;
+    line: i32;
+    col: i32;
     func: string;
     line_text: string;    // the source line, for the snippet + caret
 }
 
 // `n` spaces — used to position the `^` caret under a column.
-spaces :: (n: s32) -> string {
+spaces :: (n: i32) -> string {
     s := "";
-    i : s32 = 0;
+    i : i32 = 0;
     while i < n {
         s = concat(s, " ");
         i = i + 1;
@@ -54,7 +54,7 @@ sx_trace_len       :: () -> u32 #foreign;
 sx_trace_truncated :: () -> u32 #foreign;
 sx_trace_frame_at  :: (i: u32) -> u64 #foreign;
 
-write :: (fd: s32, buf: [*]u8, count: usize) -> isize #foreign libc;
+write :: (fd: i32, buf: [*]u8, count: usize) -> isize #foreign libc;
 
 // Render the current trace buffer to a string (allocated from
 // context.allocator). Empty buffer → "" so callers can cheaply skip output.