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/0504-packs-pack-impl-match.sx

@@ -15,10 +15,10 @@
 // `$args` and `$R` to the concrete source closure's tail types
 // and return; monomorphisation proceeds against those bindings.
 //
-// The `Closure(s32, bool) -> bool` shape is not covered by
+// The `Closure(i32, bool) -> bool` shape is not covered by
 // stdlib's hand-rolled impls (only `Closure() -> void` and
 // `Closure(bool) -> void`) and not covered by 96-block-multi-arg's
-// `Closure(s32, *void) -> void` impl, so the pack impl is the
+// `Closure(i32, *void) -> void` impl, so the pack impl is the
 // only candidate.
 
 #import "modules/std.sx";
@@ -38,8 +38,8 @@ impl Into(Block) for Closure(..$args) -> $R {
     }
 }
 
-main :: () -> s32 {
-    cl := (a: s32, b: bool) => true;
+main :: () -> i32 {
+    cl := (a: i32, b: bool) => true;
     b : *Block = xx cl;
     print("pack impl match ok\n");
     return 0;