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/render.sx

@@ -35,12 +35,12 @@ RenderNode :: struct {
     // Opacity
     opacity: f32;
 
-    depth: s64;
+    depth: i64;
 }
 
 RenderTree :: struct {
     nodes: List(RenderNode);
-    generation: s64;
+    generation: i64;
 
     init :: () -> RenderTree {
         RenderTree.{ generation = 0 }
@@ -51,7 +51,7 @@ RenderTree :: struct {
         self.generation += 1;
     }
 
-    add :: (self: *RenderTree, node: RenderNode) -> s64 {
+    add :: (self: *RenderTree, node: RenderNode) -> i64 {
         idx := self.nodes.len;
         self.nodes.append(node);
         idx
@@ -62,9 +62,9 @@ RenderTree :: struct {
 
 RenderContext :: struct {
     tree: *RenderTree;
-    clip_depth: s64;
+    clip_depth: i64;
     opacity: f32;
-    depth: s64;
+    depth: i64;
 
     init :: (tree: *RenderTree) -> RenderContext {
         RenderContext.{