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/0411-protocols-impl-duplicate.sx

@@ -5,8 +5,8 @@
 // not silently pick one or the other.
 //
 // Setup:
-//   - 180-impl-duplicate-impl-a.sx: `impl Into(Wrap) for s64` (mul by 10).
-//   - 180-impl-duplicate-impl-b.sx: `impl Into(Wrap) for s64` (add 100).
+//   - 180-impl-duplicate-impl-a.sx: `impl Into(Wrap) for i64` (mul by 10).
+//   - 180-impl-duplicate-impl-b.sx: `impl Into(Wrap) for i64` (add 100).
 //   - Main imports both; the `xx 7 : Wrap` site must error.
 //
 // Expected exit = 1, expected output = the focused diagnostic naming
@@ -19,7 +19,7 @@
 #import "./0411-protocols-impl-duplicate-impl-a.sx";
 #import "./0411-protocols-impl-duplicate-impl-b.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
   w : Wrap = xx 7;
   print("w.v = {}\n", w.v);
   0