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/1001-errors-set-typing.stderr

@@ -1,11 +1,11 @@
 error: error tag 'error.NotInSet' is not in error set 'ParseErr'
-  --> /Users/agra/projects/sx/examples/1001-errors-set-typing.sx:13:20
+  --> examples/1001-errors-set-typing.sx:13:20
    |
 13 |     c : ParseErr = error.NotInSet;   // error: NotInSet not in ParseErr
    |                    ^^^^^^^^^^^^^^
 
 error: an error-set value compares only with an `error.X` tag or another error-set value; coerce with `xx` to compare the raw id
-  --> /Users/agra/projects/sx/examples/1001-errors-set-typing.sx:14:8
+  --> examples/1001-errors-set-typing.sx:14:8
    |
 14 |     if c == 42 { return 1; }         // error: error-set value vs raw integer
    |        ^