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/0827-protocols-param-impl-source-wrapped-ambiguous/a.sx

@@ -1,8 +1,8 @@
 // One of two flat-imported `Box` authors; with no own author the param-impl
 // SOURCE `*Box` is a genuine collision.
-Box :: struct { a: s32; }
+Box :: struct { a: i32; }
 
-a_box :: () -> s32 {
+a_box :: () -> i32 {
     b : Box = ---;
     b.a = 1;
     return b.a;

examples/0827-protocols-param-impl-source-wrapped-ambiguous/b.sx

@@ -1,7 +1,7 @@
 // The second flat-imported `Box` author — a DISTINCT nominal from a.sx's `Box`.
-Box :: struct { b: s32; }
+Box :: struct { b: i32; }
 
-b_box :: () -> s32 {
+b_box :: () -> i32 {
     x : Box = ---;
     x.b = 2;
     return x.b;