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/0838-memory-helpers.sx

@@ -10,7 +10,7 @@
 #import "modules/std.sx";
 #import "modules/std/mem.sx";
 
-Session :: struct { id: s64; score: s64; }
+Session :: struct { id: i64; score: i64; }
 
 main :: () {
     gpa := GPA.init();
@@ -36,7 +36,7 @@ main :: () {
     context.allocator.destroy(q);
 
     // alloc / free — typed slice
-    xs := a |> alloc(s64, 4);
+    xs := a |> alloc(i64, 4);
     xs[0] = 10; xs[1] = 20; xs[2] = 30; xs[3] = 40;
     print("alloc: {} {} len={}\n", xs[0], xs[3], xs.len);
 
@@ -57,10 +57,10 @@ main :: () {
 
     // mem_realloc — bytes level
     raw := a.alloc_bytes(8);
-    q : *s64 = xx raw;
+    q : *i64 = xx raw;
     q.* = 1234;
     raw2 := mem_realloc(a, raw, 8, 16, 8);
-    q2 : *s64 = xx raw2;
+    q2 : *i64 = xx raw2;
     print("realloc: {}\n", q2.*);
     a.dealloc_bytes(raw2);