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/0788-modules-same-name-const-expr-chain-dim/a.sx

@@ -2,8 +2,8 @@
 // read and the array dimension must resolve `K` through A's `M`.
 M :: 1;
 K :: M + 1;
-a_val :: () -> s64 { return K; }
-a_len :: () -> s64 {
+a_val :: () -> i64 { return K; }
+a_len :: () -> i64 {
     arr : [K]u8 = ---;
     return arr.len;
 }

examples/0788-modules-same-name-const-expr-chain-dim/b.sx

@@ -3,8 +3,8 @@
 // so the dimension fold gives B's length 11 — never A's via the global map.
 M :: 10;
 K :: M + 1;
-b_val :: () -> s64 { return K; }
-b_len :: () -> s64 {
+b_val :: () -> i64 { return K; }
+b_len :: () -> i64 {
     arr : [K]u8 = ---;
     return arr.len;
 }