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/0022-basic-for-range.sx

@@ -9,7 +9,7 @@
 Show :: protocol {
     show :: () -> string;
 }
-A :: struct { x: s64; }
+A :: struct { x: i64; }
 B :: struct { s: string; }
 impl Show for A { show :: (self: *A) -> string => "A"; }
 impl Show for B { show :: (self: *B) -> string => "B"; }
@@ -21,7 +21,7 @@ each :: (..xs: Show) -> void {
     }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // Runtime range, cursor used.
     for 0..3 (i) { print("i={}\n", i); }