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.sx

@@ -84,7 +84,7 @@ checker_board :: () -> Board {
     b
 }
 
-main :: () -> s32 {
+main :: () -> i32 {
     print("== cascade (resolution loop) ==\n");
 
     // Drive the loop one round at a time so each post-round board is visible in
@@ -94,7 +94,7 @@ main :: () -> s32 {
     out(board_dump(@b));
 
     depth := 0;
-    counts := List(s64).{};
+    counts := List(i64).{};
     while true {
         n := resolve_step(@b);
         if n == 0 { break; }