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.

issues/0126-array-arg-slice-generic-param-unbound.md

@@ -30,11 +30,11 @@ during emission instead of compiling (or diagnosing).
   (src/backend/llvm/types.zig:175, via `emitIndexGet` in the
   monomorphized body).
 - **Expected**: the array coerces to a slice at the `[]T` param — the
-  same promotion a CONCRETE `[]s64` param (and a `[]s64`-annotated
+  same promotion a CONCRETE `[]i64` param (and a `[]i64`-annotated
   local) already performs — so `T` binds from the array's element type
   and the call compiles.
 
-Passing an actual slice works (`s : []s64 = a; first(s)` prints the
+Passing an actual slice works (`s : []i64 = a; first(s)` prints the
 element); only the direct array spelling breaks, and only for generic
 slice params.
 
@@ -47,8 +47,8 @@ first :: (xs: []$T) -> T {
     return xs[0];
 }
 
-main :: () -> s32 {
-    a : [3]s64 = ---;
+main :: () -> i32 {
+    a : [3]i64 = ---;
     a[0] = 7; a[1] = 8; a[2] = 9;
     v := first(a);
     print("{}\n", v);
@@ -56,7 +56,7 @@ main :: () -> s32 {
 }
 ```
 
-Observed at master 837b5d3: the panic above. With `s : []s64 = a;
+Observed at master 837b5d3: the panic above. With `s : []i64 = a;
 first(s)` it prints `7`.
 
 ## Investigation prompt