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/0211-generics-struct-alias-head.sx

@@ -19,12 +19,12 @@ ChainAlias :: LocalAlias;
 
 main :: () {
     // Same-file alias: instantiation + field + method.
-    b := LocalAlias(s64).{ item = 3 };
+    b := LocalAlias(i64).{ item = 3 };
     print("field: {}\n", b.item);
     print("method: {}\n", b.get());
 
     // Alias chain terminates at the template.
-    c := ChainAlias(s64).{ item = 11 };
+    c := ChainAlias(i64).{ item = 11 };
     print("chain: {}\n", c.item);
 
     // Alias as a type annotation head.
@@ -36,7 +36,7 @@ main :: () {
     print("helper: {}\n", helper());
     t := Thing.init();
     print("thing: {}\n", t.v);
-    f := Box(s64).{ item = 7 };
+    f := Box(i64).{ item = 7 };
     print("facade: {}\n", f.get());
     x : Box(string) = .{ item = "qq" };
     print("facade-annot: {}\n", x.item);