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.

library/modules/ffi/objc_block.sx

@@ -32,8 +32,8 @@
 // it wraps. Total = 48 bytes.
 Block :: struct {
     isa: *void;
-    flags: s32;
-    reserved: s32;
+    flags: i32;
+    reserved: i32;
     invoke: *void;
     descriptor: *void;
     sx_env: *void;
@@ -95,7 +95,7 @@ impl Into(Block) for Closure(..$args) -> $R {
 build_block_convert :: (args: []Type, $ret: Type) -> string {
     ret_name := type_name(ret);
     code := "__invoke :: (block_self: *Block";
-    i : s64 = 0;
+    i : i64 = 0;
     while i < args.len {
         code = concat(code, ", arg");
         code = concat(code, int_to_string(i));