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/1002-errors-raise.sx

@@ -9,15 +9,15 @@
 ParseErr :: error { BadDigit, Overflow, Empty }
 
 // Pure failable: raises on bad input, otherwise succeeds (error slot 0).
-check :: (n: s32) -> !ParseErr {
+check :: (n: i32) -> !ParseErr {
     if n < 0 { raise error.BadDigit; }
     return;                                 // success — no error
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     good := check(7);                       // success path  -> no error
     bad  := check(-1);                      // raise path    -> BadDigit
-    r : s32 = 0;
+    r : i32 = 0;
     if bad  == error.BadDigit { r = r + 8; }    // true  -> +8
     if good == error.BadDigit { r = r + 1; }     // false (success = no error)
     if bad  == error.Overflow { r = r + 2; }     // false (raised BadDigit)