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/expected/1003-errors-raise-rejections.stderr

@@ -1,17 +1,17 @@
 error: error tag 'error.NotInSet' is not in error set 'ParseErr'
-  --> /Users/agra/projects/sx/examples/1003-errors-raise-rejections.sx:17:11
+  --> examples/1003-errors-raise-rejections.sx:17:11
    |
 17 |     raise error.NotInSet;            // error: NotInSet not in ParseErr
    |           ^^^^^^^^^^^^^^
 
 error: error tag 'error.Weird' is not in caller's error set 'ParseErr'
-  --> /Users/agra/projects/sx/examples/1003-errors-raise-rejections.sx:24:5
+  --> examples/1003-errors-raise-rejections.sx:24:5
    |
 24 |     raise e;                         // error: OtherErr not subset of ParseErr
    |     ^^^^^^^^
 
 error: `raise` is only valid inside a failable function (a return type with `!` or `!Named`)
-  --> /Users/agra/projects/sx/examples/1003-errors-raise-rejections.sx:30:5
+  --> examples/1003-errors-raise-rejections.sx:30:5
    |
-30 |     raise error.BadDigit;            // error: main (-> s32) is not failable
+30 |     raise error.BadDigit;            // error: main (-> i32) is not failable
    |     ^^^^^^^^^^^^^^^^^^^^^