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/lower/nominal.zig

@@ -102,7 +102,7 @@ pub fn reserveShadowEnumSlot(self: *Lowering, ed: *const ast.EnumDecl) void {
     const name_id = table.internString(ed.name);
     const nominal_id = self.shadowNominalId(name_id);
     const empty: types.TypeInfo = if (ed.variant_types.len > 0)
-        .{ .tagged_union = .{ .name = name_id, .fields = &.{}, .tag_type = .s64 } }
+        .{ .tagged_union = .{ .name = name_id, .fields = &.{}, .tag_type = .i64 } }
     else
         .{ .@"enum" = .{ .name = name_id, .variants = &.{} } };
     const reserved = table.internNominal(empty, nominal_id);
@@ -534,7 +534,7 @@ pub fn bareVisibleStructTemplate(self: *Lowering, name: []const u8) ?StructTempl
 }
 
 /// Instantiate a generic struct template and register the result under an
-/// alias name (`Vec3 :: Vec(3, f32)` / `ABox :: a.Box(s64)`). Shared by the
+/// alias name (`Vec3 :: Vec(3, f32)` / `ABox :: a.Box(i64)`). Shared by the
 /// `.call` and `.parameterized_type_expr` const-decl alias branches and the
 /// qualified-head selection that precedes the bare `struct_template_map`
 /// fallback in each.