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/0071-global-initializer-module-const-silent-zero.md

@@ -35,12 +35,12 @@ supported.
 #import "modules/std.sx";
 
 A :: B;
-B :: s32;
+B :: i32;
 
 K : A : 42;
 g : A = K;
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("g={}\n", g);
     return g;
 }
@@ -82,7 +82,7 @@ Likely fix:
   the global's declared type.
 - If some initializer shape cannot be represented as a global constant yet, emit
   a diagnostic instead of returning `null` / zero-initializing.
-- Do not regress issue 0070: `A :: B; B :: s32; g : A = 7;` and
+- Do not regress issue 0070: `A :: B; B :: i32; g : A = 7;` and
   `K : A : 35;` must still resolve through the converged alias map.
 - Preserve literal, array literal, struct literal, and foreign-global behavior.
 
@@ -92,10 +92,10 @@ Verification:
 ```sx
 #import "modules/std.sx";
 A :: B;
-B :: s32;
+B :: i32;
 K : A : 42;
 g : A = K;
-main :: () -> s32 { print("g={}\n", g); return g; }
+main :: () -> i32 { print("g={}\n", g); return g; }
 ```
 
 - Keep these green: