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/0792-modules-same-name-const-struct-field-dim/a.sx

@@ -2,4 +2,4 @@
 // by A's `K`. `size_of(Box)` folds the field dimension against A's `K` (= 2).
 K :: 2;
 Box :: struct { arr: [K]u8; }
-a_sz :: () -> s64 { return size_of(Box); }
+a_sz :: () -> i64 { return size_of(Box); }

examples/0792-modules-same-name-const-struct-field-dim/b.sx

@@ -3,4 +3,4 @@
 // differs from A's — never collapsed to a global last-wins `K`.
 K :: 7;
 Box :: struct { arr: [K]u8; }
-b_sz :: () -> s64 { return size_of(Box); }
+b_sz :: () -> i64 { return size_of(Box); }