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/0052-basic-slice-range-bounds.sx

@@ -6,15 +6,15 @@
 
 #import "modules/std.sx";
 
-dump :: (s: []s64, tag: string) {
+dump :: (s: []i64, tag: string) {
     print("{}: ", tag);
     for s (v) { print("{} ", v); }
     print("(len {})\n", s.len);
 }
 
-main :: () -> s32 {
-    xs : [6]s64 = .[10, 11, 12, 13, 14, 15];
-    full : []s64 = xs[0..6];
+main :: () -> i32 {
+    xs : [6]i64 = .[10, 11, 12, 13, 14, 15];
+    full : []i64 = xs[0..6];
 
     dump(full[1..=3],  "1..=3");     // 11 12 13
     dump(full[0<..<4], "0<..<4");    // 11 12 13