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/0731-modules-flat-same-name-closure.sx

@@ -1,7 +1,7 @@
 // fix-0102d site 2 (issue 0102): two flat FILE imports each author a same-name
 // free function `pick` (a.sx returns 1, b.sx returns 2). Each module takes
 // `pick` as a function VALUE — both as `closure(pick)` and as a bare-name
-// fn-pointer binding (`g : () -> s64 = pick`). The captured FuncId must be the
+// fn-pointer binding (`g : () -> i64 = pick`). The captured FuncId must be the
 // RESOLVED author's (own-author wins), not the first-wins winner's. Before the
 // fix, b.sx's `closure(pick)` / `pick`-as-value both captured a.sx's winner
 // (1). Regression: per-source function-value conversion (closure + func_ref).
@@ -13,7 +13,7 @@ report :: (label: string, ok: bool) {
     if ok { print("{}: ok\n", label); } else { print("{}: FAIL\n", label); }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     report("from_a closure binds a.pick (1)", from_a_closure() == 1);
     report("from_b closure binds b.pick (2)", from_b_closure() == 2);
     report("from_a fn-value binds a.pick (1)", from_a_value() == 1);