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/dock.sx

@@ -93,12 +93,12 @@ DOCK_ANIM_DURATION :f32: 0.19;  // 190ms
 
 DockInteraction :: struct {
     // Drag state
-    dragging_child: s64;       // -1 = none
+    dragging_child: i64;       // -1 = none
     drag_start_pos: Point;
     drag_offset: Point;
     click_fraction_x: f32;
     click_fraction_y: f32;
-    hovered_zone: s64;         // -1 = none, else DockZone ordinal
+    hovered_zone: i64;         // -1 = none, else DockZone ordinal
 
     // Per-child state
     natural_sizes: List(Size);
@@ -111,7 +111,7 @@ DockInteraction :: struct {
     anim_sizes: List(Animated(Size));
     header_pressed: List(bool);
 
-    child_count: s64;
+    child_count: i64;
     parent_allocator: Allocator;  // GPA — used for persistent list growth
 
     init :: (self: *DockInteraction) {
@@ -135,7 +135,7 @@ DockInteraction :: struct {
         self.header_pressed = List(bool).{};
     }
 
-    ensure_capacity :: (self: *DockInteraction, count: s64) {
+    ensure_capacity :: (self: *DockInteraction, count: i64) {
         if self.child_count >= count { return; }
         while self.child_count < count {
             self.natural_sizes.append(Size.zero(), self.parent_allocator);
@@ -151,7 +151,7 @@ DockInteraction :: struct {
         }
     }
 
-    set_target_size :: (self: *DockInteraction, index: s64, target: Size) {
+    set_target_size :: (self: *DockInteraction, index: i64, target: Size) {
         if index >= self.child_count { return; }
         anim := @self.anim_sizes.items[index];
         cur := anim.to;
@@ -168,7 +168,7 @@ DockInteraction :: struct {
         }
     }
 
-    get_animated_size :: (self: *DockInteraction, index: s64) -> Size {
+    get_animated_size :: (self: *DockInteraction, index: i64) -> Size {
         if index >= self.child_count { return Size.zero(); }
         (@self.anim_sizes.items[index]).current
     }
@@ -197,7 +197,7 @@ DockInteraction :: struct {
     }
 }
 
-start_dragging :: (interaction: *DockInteraction, child_index: s64, pos: Point, panel_frame: Frame) {
+start_dragging :: (interaction: *DockInteraction, child_index: i64, pos: Point, panel_frame: Frame) {
     interaction.dragging_child = child_index;
     interaction.drag_start_pos = pos;
     interaction.drag_offset = Point.zero();
@@ -218,7 +218,7 @@ start_dragging :: (interaction: *DockInteraction, child_index: s64, pos: Point,
 // Helper functions
 // =============================================================================
 
-zone_by_index :: (i: s64) -> DockZone {
+zone_by_index :: (i: i64) -> DockZone {
     if i == {
         case 0: .fill;
         case 1: .center;
@@ -234,8 +234,8 @@ zone_by_index :: (i: s64) -> DockZone {
 }
 
 find_hovered_zone :: (bounds: Frame, pos: Point, hint_size: f32, enable_corners: bool) -> ?DockZone {
-    count : s64 = if enable_corners then 10 else 6;
-    i : s64 = 0;
+    count : i64 = if enable_corners then 10 else 6;
+    i : i64 = 0;
     while i < count {
         zone := zone_by_index(i);
         hint := dock_zone_get_hint_frame(zone, bounds, hint_size);
@@ -365,7 +365,7 @@ DockPanel :: struct {
     corner_radius: f32;
     header_height: f32;
     dock_interaction: *DockInteraction;
-    panel_index: s64;
+    panel_index: i64;
 
     DEFAULT_BG        :Color: Color.rgba(26, 26, 31, 242);
     DEFAULT_HEADER_BG  :Color: Color.rgba(38, 38, 46, 255);
@@ -469,7 +469,7 @@ Dock :: struct {
     hint_active_color: Color;
     preview_color: Color;
     enable_corners: bool;
-    on_dock: ?Closure(s64, DockZone);
+    on_dock: ?Closure(i64, DockZone);
 
     make :: (interaction: *DockInteraction, delta_time: *f32) -> Dock {
         d : Dock = ---;
@@ -519,7 +519,7 @@ impl View for Dock {
         dt : f32 = self.delta_time.*;
         interaction.tick_animations(dt);
 
-        i : s64 = 0;
+        i : i64 = 0;
         while i < self.children.len {
             child := @self.children.items[i];
 
@@ -603,7 +603,7 @@ impl View for Dock {
         }
 
         // Draw children
-        i : s64 = 0;
+        i : i64 = 0;
         while i < self.children.len {
             child := @self.children.items[i];
             child.view.render(ctx, child.computed_frame);
@@ -619,8 +619,8 @@ impl View for Dock {
             }
 
             // Zone hint indicators
-            count : s64 = if self.enable_corners then 10 else 6;
-            j : s64 = 0;
+            count : i64 = if self.enable_corners then 10 else 6;
+            j : i64 = 0;
             while j < count {
                 zone := zone_by_index(j);
                 hint_frame := dock_zone_get_hint_frame(zone, frame, self.hint_size);