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/1006-errors-inferred-error-sets.sx

@@ -11,28 +11,28 @@
 
 A :: error { Foo, Bar }
 
-leaf :: (n: s32) -> ! {
+leaf :: (n: i32) -> ! {
     if n < 0 { raise error.Foo; }
     return;
 }
 
 // Inferred set converges to {Foo, Bar}: {Foo} absorbed from `try leaf` plus
 // the directly-raised Bar.
-mid :: (n: s32) -> ! {
+mid :: (n: i32) -> ! {
     try leaf(n);
     if n == 100 { raise error.Bar; }
     return;
 }
 
 // Named caller: mid's converged {Foo, Bar} is a subset of A -> widening OK.
-run :: (n: s32) -> !A {
+run :: (n: i32) -> !A {
     try mid(n);
     return;
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     e := run(-1);                          // leaf raises Foo -> propagates out
-    r : s32 = 0;
+    r : i32 = 0;
     if e == error.Foo { r = r + 7; }       // true  -> +7
     if e == error.Bar { r = r + 1; }        // false (Foo escaped, not Bar)
     print("inferred result: {}\n", r);      // -> 7