lang migration: rename signed integer types sN -> iN

Mechanical sweep of all .sx sources, plan docs, and tests/expected
snapshots for the sx language rename (s8/s16/s32/s64 -> i8/i16/i32/i64).
Verified: tools/run_tests.sh 23/23.

Note: the ios-sim build has 2 pre-existing 'restart' dot-call errors
from the sx opt-in UFCS change (sx a47ea14) — independent of this
rename (present pre-sweep); migrated in the follow-up commit.

tests/fx_combo.sx

@@ -13,14 +13,14 @@
 #import "modules/std.sx";
 #import "board_fx.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== combo emphasis selection (depth -> fx level / popup font) ==\n");
 
     // The cascade-cue index per depth 0..9, copied from cascade_cue.stdout. The
     // FX level must equal this entry for entry — the audio/visual lockstep.
-    expect_level : [10]s64 = .{ 0, 0, 1, 2, 3, 4, 4, 4, 4, 4 };
+    expect_level : [10]i64 = .{ 0, 0, 1, 2, 3, 4, 4, 4, 4, 4 };
 
-    prev : s64 = -1;
+    prev : i64 = -1;
     for 0..10 (depth) {
         lvl := fx_combo_level(depth);
         font := fx_popup_font(depth);