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/0773-modules-qualified-generic-alias-author.sx

@@ -2,7 +2,7 @@
 // instantiate the template AUTHORED by `ns`'s module — not the global same-name
 // template that won the last-wins `struct_template_map`. Two namespaces each
 // author a same-name generic `Box($T)` with a DIFFERENT layout (a: one field,
-// b: two fields). `ABox :: a.Box(s64)` and `BBox :: b.Box(s64)` must register
+// b: two fields). `ABox :: a.Box(i64)` and `BBox :: b.Box(i64)` must register
 // aliases over their OWN module's template (sizes 8 and 16) and stay DISTINCT,
 // so the field unique to b's layout (`y`) is reachable only through `BBox`.
 //
@@ -16,10 +16,10 @@
 a :: #import "0773-modules-qualified-generic-alias-author/a.sx";
 b :: #import "0773-modules-qualified-generic-alias-author/b.sx";
 
-ABox :: a.Box(s64);
-BBox :: b.Box(s64);
+ABox :: a.Box(i64);
+BBox :: b.Box(i64);
 
-main :: () -> s32 {
+main :: () -> i32 {
     ab : ABox = .{ x = 1 };
     bb : BBox = .{ x = 10, y = 20 };
     print("alias a={} b={}\n", size_of(ABox), size_of(BBox));