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/0038-basic-dead-code-after-terminator.sx

@@ -14,15 +14,15 @@
 E :: error { Neg }
 
 // dead `return 99;` after an unconditional return
-const_one :: () -> s64 { return 1; return 99; }
+const_one :: () -> i64 { return 1; return 99; }
 
 // dead `return x;` after an unconditional raise (the failable closure shape)
-always_raise :: (x: s64) -> (s64, !E) { raise error.Neg; return x; }
+always_raise :: (x: i64) -> (i64, !E) { raise error.Neg; return x; }
 
 // guard: a conditional return must still fall through to the trailing return
-clamp :: (x: s64) -> s64 { if x > 10 { return 10; } return x; }
+clamp :: (x: i64) -> i64 { if x > 10 { return 10; } return x; }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("const_one={}\n", const_one());            // 1
     print("raised={}\n", always_raise(5) catch (e) 0); // 0
     print("clamp_hi={}\n", clamp(42));               // 10