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/0518-packs-pack-value-dispatch.sx

@@ -8,7 +8,7 @@
 // 1. `type_name($args[0])` — the value-form Type goes through the
 //    reflection intrinsic, picking up the per-mono concrete type.
 //
-// 2. `inline if type_eq($args[0], s64) { ... }` — compile-time
+// 2. `inline if type_eq($args[0], i64) { ... }` — compile-time
 //    branch over the pack element's type. The fold via
 //    `tryConstBoolCondition` reads $args[0] via `resolveTypeArg`,
 //    which the step-4 work taught to walk `pack_arg_types`.
@@ -22,20 +22,20 @@
 show :: (..$args) -> string => type_name($args[0]);
 
 describe :: (..$args) -> string {
-    inline if type_eq($args[0], s64) { return "got s64"; }
+    inline if type_eq($args[0], i64) { return "got i64"; }
     inline if type_eq($args[0], string) { return "got string"; }
     inline if type_eq($args[0], bool) { return "got bool"; }
     return "got other";
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // type_name picks up the per-mono first-arg type.
-    print("{}\n", show(42));         // s64
+    print("{}\n", show(42));         // i64
     print("{}\n", show("hi"));        // string
     print("{}\n", show(3.14));        // f64
 
     // inline-if + type_eq picks the right branch per mono.
-    print("{}\n", describe(42));      // got s64
+    print("{}\n", describe(42));      // got i64
     print("{}\n", describe("hello"));  // got string
     print("{}\n", describe(true));     // got bool
     print("{}\n", describe(3.14));     // got other