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/0050-basic-for-multi-iterable.sx

@@ -1,29 +1,29 @@
 #import "modules/std.sx";
 
-pair_sum :: (xs: []s64, ys: []s64) -> s64 {
+pair_sum :: (xs: []i64, ys: []i64) -> i64 {
     total := 0;
     for xs, ys (x, y) { total += x * y; }
     total
 }
 
-make :: () -> [3]s64 {
-    r : [3]s64 = .[7, 8, 9];
+make :: () -> [3]i64 {
+    r : [3]i64 = .[7, 8, 9];
     r
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     // Agra's example: a 1..5 inclusive, b open-ended following along.
     for 1..=5, 0.. (a, b) { print("{}:{} ", a, b); }
     print("\n");
 
     // Index idiom replacing the old (x, i) form.
-    xs : [3]s64 = .[10, 20, 30];
+    xs : [3]i64 = .[10, 20, 30];
     for xs, 0.. (x, i) { print("[{}]={} ", i, x); }
     print("\n");
 
     // Parallel slices.
-    a4 : [4]s64 = .[1, 2, 3, 4];
-    b4 : [4]s64 = .[10, 20, 30, 40];
+    a4 : [4]i64 = .[1, 2, 3, 4];
+    b4 : [4]i64 = .[10, 20, 30, 40];
     print("dot={}\n", pair_sum(a4, b4));
 
     // Arrow bodies.