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/cascade_rounds.sx

@@ -13,7 +13,7 @@
 #import "board_anim.sx";
 #import "audio.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== per-round cascade cue timing ==\n");
 
     // `cascade_rounds_started` = how many cascade rounds have BEGUN clearing by
@@ -21,7 +21,7 @@ main :: () -> s32 {
     // Round k (0-based) starts clearing at 0.16 + k*0.36; sampled safely INSIDE
     // each round window so the integer step is unambiguous. Locked for 5 rounds.
     print("-- started-count across a 5-round chain --\n");
-    rounds : s64 = 5;
+    rounds : i64 = 5;
     print("e=0.00 -> {}\n", cascade_rounds_started(0.00, rounds));
     print("e=0.10 -> {}\n", cascade_rounds_started(0.10, rounds));
     print("e=0.20 -> {}\n", cascade_rounds_started(0.20, rounds));
@@ -37,7 +37,7 @@ main :: () -> s32 {
     // ascending. This IS the loop main's frame loop runs; the emitted run is the
     // locked acceptance ordering.
     print("-- ascending per-round run --\n");
-    fired : s64 = 0;
+    fired : i64 = 0;
     elapsed : f32 = 0.0;
     while fired < rounds {
         started := cascade_rounds_started(elapsed, rounds);