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/1125-diagnostics-reserved-name-method-param.sx

@@ -1,7 +1,7 @@
 // A reserved/builtin type name used as a PARAMETER name is rejected inside the
 // two method-with-body forms that carry their params as bare name lists rather
 // than `Param` nodes: a protocol default-body method (`u8`) and a sx-defined
-// foreign-class (`#objc_class`) method (`s16`). The declaration-site diagnostic
+// foreign-class (`#objc_class`) method (`i16`). The declaration-site diagnostic
 // underlines the OFFENDING PARAMETER itself, not the enclosing `protocol` /
 // `#objc_class` block — each method's `param_name_spans` is threaded from the
 // parser so the caret lands on the parameter token.
@@ -12,19 +12,19 @@
 #import "modules/build.sx";
 
 Greeter :: protocol {
-    greet :: (self: *Self, u8: s64) -> s64 {
+    greet :: (self: *Self, u8: i64) -> i64 {
         return u8;
     }
 }
 
 SxFoo :: #objc_class("SxFoo") {
-    counter: s32;
+    counter: i32;
 
-    bump :: (self: *Self, s16: s32) {
-        self.counter += s16;
+    bump :: (self: *Self, i16: i32) {
+        self.counter += i16;
     }
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     return 0;
 }