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/1011-errors-value-failable.sx

@@ -10,14 +10,14 @@
 
 E :: error { Bad, Empty }
 
-parse :: (n: s32) -> (s32, !E) {
+parse :: (n: i32) -> (i32, !E) {
     if n < 0  { raise error.Bad; }
     if n == 0 { raise error.Empty; }
     return n * 10;                       // success → {n*10, 0}
 }
 
-main :: () -> s32 {
-    r : s32 = 0;
+main :: () -> i32 {
+    r : i32 = 0;
 
     // The value slot is live only where the error is proven absent (ERR E1.8):
     // read `v1` under an `if !e1` guard, not after a bare tag-compare.