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/0053-basic-ufcs-opt-in.sx

@@ -8,17 +8,17 @@
 
 #import "modules/std.sx";
 
-bump  :: (x: s64) -> s64 { x + 1 }
-bump2 :: ufcs (x: s64) -> s64 { x + 2 }
+bump  :: (x: i64) -> i64 { x + 1 }
+bump2 :: ufcs (x: i64) -> i64 { x + 2 }
 bump3 :: ufcs bump;
 
-Counter :: struct { n: s64; }
-inc :: ufcs (c: *Counter, by: s64) -> s64 { c.n += by; c.n }
+Counter :: struct { n: i64; }
+inc :: ufcs (c: *Counter, by: i64) -> i64 { c.n += by; c.n }
 
 gfirst :: ufcs (xs: []$T) -> T { xs[0] }
 
 main :: () {
-    f : s64 = 40;
+    f : i64 = 40;
     print("marked: {}\n", f.bump2());      // 42
     print("alias: {}\n", f.bump3());       // 41
     print("direct: {}\n", bump(f));        // 41 — plain fn, direct
@@ -29,7 +29,7 @@ main :: () {
     print("ptr-recv: {}\n", c.inc(5));     // 15 — auto address-of receiver
 
     arr := .[7, 8, 9];
-    xs : []s64 = arr;
+    xs : []i64 = arr;
     print("generic-dot: {}\n", xs.gfirst());     // 7
-    print("generic-direct: {}\n", gfirst(xs));   // 7 — plan types it s64, not a T stub
+    print("generic-direct: {}\n", gfirst(xs));   // 7 — plan types it i64, not a T stub
 }