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/0124-large-stack-array-aggregate-ops-crash-llvm.md

@@ -16,7 +16,7 @@
 > (`any_to_string`'s per-array-type arms pass the array by value — any
 > 64K+ array type + any `{}` print still crashes).
 > Regression test: `examples/0055-basic-large-stack-array.sx`
-> ([65536]u8 write/read loops + [131072]s64 first/last — `sx build`
+> ([65536]u8 write/read loops + [131072]i64 first/last — `sx build`
 > segfaulted pre-fix). 22 `.ir` snapshots re-pinned (removed undef
 > stores / `ig.tmp` spills → in-place gep+load; reviewed
 > instruction-shape-only). Gates: zig build test 426/426, suite
@@ -59,12 +59,12 @@ replaced by in-place access the module compiles.
 ```sx
 #import "modules/std.sx";
 
-f :: (fd: s32) {
+f :: (fd: i32) {
     buf : [65536]u8 = ---;
     if buf[0] > 0 { out("x\n"); }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     f(1);
     return 0;
 }