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/0783-modules-qualified-generic-alias-instance-dispatch.sx

@@ -1,8 +1,8 @@
-// A generic-struct ALIAS whose RHS is a qualified head (`ABox :: a.Box(s64)`)
+// A generic-struct ALIAS whose RHS is a qualified head (`ABox :: a.Box(i64)`)
 // must make an ALIAS-typed receiver (`x: ABox`) a first-class dispatch instance:
 // `x.tag()` runs the author `a`'s body with `a`'s bindings — never a dead end.
 // Two namespaces author a same-name generic `Box($T)` with a DIFFERENT layout and
-// a DIFFERENT `tag`; the alias over `a.Box(s64)` must dispatch `a.Box.tag`.
+// a DIFFERENT `tag`; the alias over `a.Box(i64)` must dispatch `a.Box.tag`.
 //
 // Regression (Phase E4 Counter-2): the alias registration cloned the layout into
 // a fresh type named `ABox` but did NOT mirror the instance template/bindings/
@@ -14,9 +14,9 @@
 a :: #import "0783-modules-qualified-generic-alias-instance-dispatch/a.sx";
 b :: #import "0783-modules-qualified-generic-alias-instance-dispatch/b.sx";
 
-ABox :: a.Box(s64);
+ABox :: a.Box(i64);
 
-main :: () -> s32 {
+main :: () -> i32 {
     x : ABox = .{ x = 5 };
     print("size={} tag={}\n", size_of(ABox), x.tag());
     0