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/0541-packs-pack-to-protocol-tuple.sx

@@ -6,9 +6,9 @@
 #import "modules/std.sx";
 
 VL :: protocol(T: Type) { get :: () -> T; }
-IntCell :: struct { v: s64; }
+IntCell :: struct { v: i64; }
 StrCell :: struct { s: string; }
-impl VL(s64) for IntCell { get :: (self: *IntCell) -> s64 => self.v; }
+impl VL(i64) for IntCell { get :: (self: *IntCell) -> i64 => self.v; }
 impl VL(string) for StrCell { get :: (self: *StrCell) -> string => self.s; }
 
 Combined :: struct($R: Type, ..$Ts: []Type) {
@@ -17,12 +17,12 @@ Combined :: struct($R: Type, ..$Ts: []Type) {
 }
 
 build :: (..sources: VL) -> void {
-    c : Combined(s64, ..sources.T) = ---;
+    c : Combined(i64, ..sources.T) = ---;
     c.sources = (..sources);                          // pack → tuple, per-element erase
     print("{} {}\n", c.sources.0.get(), c.sources.1.get());
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     build(IntCell.{ v = 10 }, StrCell.{ s = "hi" });   // 10 hi
     0
 }