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/0100-cross-module-same-name-fn-lowering-collision.md

@@ -50,7 +50,7 @@ from **its own module's flat import** was rejected:
 
 ```
 m :: #import "m.sx";            // m.sx: `#import "helper.sx"; foo :: () { helper() }`
-main :: () -> s32 { print("{}\n", m.foo()); 0 }   // → 'helper' is not visible
+main :: () -> i32 { print("{}\n", m.foo()); 0 }   // → 'helper' is not visible
 ```
 
 **Fix** (`src/ir/program_index.zig`, `src/ir/lower.zig`):
@@ -81,8 +81,8 @@ lowering and the caller's own trailing statements / `return` were treated as
 dead-after-terminator:
 
 ```
-m :: #import "m.sx";   // m.sx: `#import "helper.sx"; foo :: () -> s64 { if true { return helper(); } return 0; }`
-main :: () -> s32 {
+m :: #import "m.sx";   // m.sx: `#import "helper.sx"; foo :: () -> i64 { if true { return helper(); } return 0; }`
+main :: () -> i32 {
     x := m.foo();
     print("after\n");  // dropped
     return 0;          // → error: body produces no value
@@ -132,7 +132,7 @@ exits 0 after. 0719 and 0720 stay green.
 cli  :: #import "modules/std/cli.sx";
 json :: #import "modules/std/json.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     gpa := GPA.init();
     arena := Arena.init(xx gpa, 8192);
     defer arena.deinit();