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/0037-basic-trailing-commas.sx

@@ -4,7 +4,7 @@
 #import "modules/std/test.sx";
 pkg :: #import "tests/fixtures/testpkg";
 
-add :: (a: s32, b: s32) -> s32 { a + b }
+add :: (a: i32, b: i32) -> i32 { a + b }
 
 main :: () {
 
@@ -23,12 +23,12 @@ main :: () {
         assert(v.w == 4.0);
 
         // Function call with trailing comma
-        add :: (a: s64, b: s64) -> s64 { return a + b; }
+        add :: (a: i64, b: i64) -> i64 { return a + b; }
         r := add(10, 20,);
         assert(r == 30);
 
         // Array literal with trailing comma
-        arr := s64.[1, 2, 3,];
+        arr := i64.[1, 2, 3,];
         assert(arr[2] == 3);
 
         print("trailing commas ok\n");