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_cue.sx

@@ -8,13 +8,13 @@
 #import "modules/std.sx";
 #import "audio.sx";
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== cascade cue selection (depth -> combo cue) ==\n");
 
     // Walk a representative depth range (0..9) so both clamps and the monotonic
     // middle are visible: depths 0,1 pin to the first cue; depths >= 5 pin to
     // the last; 2,3,4 step up one cue at a time.
-    prev : s64 = -1;
+    prev : i64 = -1;
     for 0..10 (depth) {
         idx := cascade_cue_index(depth);
         print("depth {} -> idx {} ({})\n", depth, idx, cascade_cue_name(idx));