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/ui/state.sx

@@ -14,17 +14,17 @@ State :: struct ($T: Type) {
 // --- StateEntry — type-erased storage ---
 
 StateEntry :: struct {
-    id: s64;
+    id: i64;
     data: [*]u8;
-    size: s64;
-    generation: s64;
+    size: i64;
+    generation: i64;
 }
 
 // --- StateStore — manages persistent state ---
 
 StateStore :: struct {
     entries: List(StateEntry);
-    current_generation: s64;
+    current_generation: i64;
     parent_allocator: Allocator;
 
     init :: (self: *StateStore) {
@@ -33,9 +33,9 @@ StateStore :: struct {
         self.parent_allocator = context.allocator;
     }
 
-    get_or_create :: (self: *StateStore, id: s64, $T: Type, default: T) -> State(T) {
+    get_or_create :: (self: *StateStore, id: i64, $T: Type, default: T) -> State(T) {
         // Search for existing entry
-        i : s64 = 0;
+        i : i64 = 0;
         while i < self.entries.len {
             if self.entries.items[i].id == id {
                 self.entries.items[i].generation = self.current_generation;