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/1004-errors-try.sx

@@ -9,21 +9,21 @@
 
 E :: error { Bad, Worse }
 
-inner :: (n: s32) -> !E {
+inner :: (n: i32) -> !E {
     if n < 0 { raise error.Bad; }
     return;                              // success — no error
 }
 
 // Propagates inner's error (standalone `try`, target = function return).
-outer :: (n: s32) -> !E {
+outer :: (n: i32) -> !E {
     try inner(n);
     return;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     bad  := outer(-1);                   // inner raises Bad -> outer propagates
     good := outer(7);                    // inner succeeds   -> outer succeeds
-    r : s32 = 0;
+    r : i32 = 0;
     if bad  == error.Bad   { r = r + 5; }    // true  -> +5
     if good == error.Bad   { r = r + 1; }     // false (success = no error)
     if bad  == error.Worse { r = r + 2; }     // false (propagated Bad)