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/1005-errors-try-rejections.sx

@@ -13,17 +13,17 @@ B :: error { Yb }
 
 ga    :: () -> !A  { return; }
 gb    :: () -> !B  { return; }
-plain :: () -> s32 { return 0; }
+plain :: () -> i32 { return 0; }
 
 // `try` in a non-failable function.
-bad_ctx :: () -> s32 {
+bad_ctx :: () -> i32 {
     try ga();                  // error: `try` outside a failable function
     return 0;
 }
 
 // `try` on a non-failable operand.
 bad_operand :: () -> !A {
-    try plain();               // error: operand has type s32 (not failable)
+    try plain();               // error: operand has type i32 (not failable)
     return;
 }
 
@@ -33,7 +33,7 @@ widen :: () -> !A {
     return;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     a := bad_ctx();            // force bad_ctx to lower
     b := bad_operand();        // force bad_operand to lower
     c := widen();              // force widen to lower