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.

src/ir/jni_descriptor.zig

@@ -6,10 +6,10 @@
 //
 //   void          → V
 //   bool          → Z   (jboolean)
-//   s8            → B   (jbyte)
-//   s16           → S   (jshort)
-//   s32           → I   (jint)
-//   s64           → J   (jlong)
+//   i8            → B   (jbyte)
+//   i16           → S   (jshort)
+//   i32           → I   (jint)
+//   i64           → J   (jlong)
 //   u8            → B   (jbyte — JNI bytes are signed; sx u8 still bridges via B)
 //   u16           → C   (jchar — unsigned 16-bit)
 //   f32           → F   (jfloat)
@@ -137,12 +137,12 @@ fn primitiveChar(name: []const u8) ?u8 {
     const table = [_]struct { name: []const u8, ch: u8 }{
         .{ .name = "void", .ch = 'V' },
         .{ .name = "bool", .ch = 'Z' },
-        .{ .name = "s8", .ch = 'B' },
+        .{ .name = "i8", .ch = 'B' },
         .{ .name = "u8", .ch = 'B' },
-        .{ .name = "s16", .ch = 'S' },
+        .{ .name = "i16", .ch = 'S' },
         .{ .name = "u16", .ch = 'C' },
-        .{ .name = "s32", .ch = 'I' },
-        .{ .name = "s64", .ch = 'J' },
+        .{ .name = "i32", .ch = 'I' },
+        .{ .name = "i64", .ch = 'J' },
         .{ .name = "f32", .ch = 'F' },
         .{ .name = "f64", .ch = 'D' },
     };
@@ -160,7 +160,7 @@ fn primitiveChar(name: []const u8) ?u8 {
 /// Pointer-typed returns route through `CallObjectMethod`.
 pub fn isJniReturnTypeSupported(table: *const types.TypeTable, ret_ty: TypeId) bool {
     return switch (ret_ty) {
-        .void, .bool, .s32, .s64, .f32, .f64 => true,
+        .void, .bool, .i32, .i64, .f32, .f64 => true,
         else => blk: {
             if (ret_ty.isBuiltin()) break :blk false;
             const info = table.get(ret_ty);