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.

examples/0806-memory-static-method-inline-xx-protocol-arg.sx

@@ -1,6 +1,6 @@
 // Inline `xx` cast as the first argument to a struct static method must
 // flow the leading param's type into the cast — otherwise an `xx ptr`
-// targeting a protocol param falls back to s64 and the call frame is
+// targeting a protocol param falls back to i64 and the call frame is
 // corrupted, SIGTRAPping when the body dispatches through the field.
 //
 // Three call shapes that must all succeed:
@@ -15,7 +15,7 @@ Box :: struct {
     parent: Allocator;
     first_ptr: *void;
 
-    init :: (parent_alloc: Allocator, size: s64) -> *Box {
+    init :: (parent_alloc: Allocator, size: i64) -> *Box {
         self : *Box = xx libc_malloc(size_of(Box));
         self.parent = parent_alloc;
         self.first_ptr = self.parent.alloc_bytes(size);
@@ -23,14 +23,14 @@ Box :: struct {
     }
 }
 
-make_box :: (parent_alloc: Allocator, size: s64) -> *Box {
+make_box :: (parent_alloc: Allocator, size: i64) -> *Box {
     self : *Box = xx libc_malloc(size_of(Box));
     self.parent = parent_alloc;
     self.first_ptr = self.parent.alloc_bytes(size);
     self
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     g_gpa : GPA = .{ alloc_count = 0 };
 
     a : Allocator = xx @g_gpa;