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/1024-errors-trace-buffer.sx

@@ -13,17 +13,17 @@ sx_trace_len :: () -> u32 #foreign;
 
 E :: error { Bad }
 
-fail :: (n: s32) -> !E {
+fail :: (n: i32) -> !E {
     if n < 0 { raise error.Bad; }   // pushes a frame
     return;
 }
 
-propagate :: (n: s32) -> !E {
+propagate :: (n: i32) -> !E {
     try fail(n);                     // on failure: pushes a frame, propagates
     return;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // `catch` lets the handler INSPECT the trace, then absorbs: the buffer is
     // cleared when the handler completes (a non-diverging exit), not on entry.
     // So inside the handler the frames are still visible (here: the `raise` in