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/0101-types-types.sx

@@ -1,7 +1,7 @@
 #import "modules/std.sx";
 SPECIAL_VALUE :u8: 42;
 
-resolve :: (x: u8) -> s32 {
+resolve :: (x: u8) -> i32 {
     return 12 + x;
 }
 
@@ -9,7 +9,7 @@ Foo :: struct {
     a : u2; // this will have 0 as default
     b : u8 = SPECIAL_VALUE; 
     c : u8 = ---; // default for c is undefined
-    d : u8 = #run xx resolve(5); // converts s32 to u8
+    d : u8 = #run xx resolve(5); // converts i32 to u8
 }
 
 main :: () {
@@ -38,6 +38,6 @@ Pack :: struct {
     c: u8;    
     d: u32;
     f: u64;
-    v: s32;
+    v: i32;
     x: f32;
 }