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.

.vscode/tasks.json

@@ -0,0 +1,23 @@
+{
+  "version": "2.0.0",
+  "tasks": [
+    {
+      "label": "sx build (macos debug)",
+      "detail": "Build M3te for macOS with DWARF (-O0) and collect a .dSYM for lldb.",
+      "type": "shell",
+      "command": "/Users/agra/projects/sx/zig-out/bin/sx build --target macos --emit-obj main.sx && /opt/homebrew/opt/llvm@19/bin/dsymutil sx-out/macos/M3te.app/Contents/MacOS/M3te",
+      "options": { "cwd": "${workspaceFolder}" },
+      "group": "build",
+      "problemMatcher": []
+    },
+    {
+      "label": "sx build+launch (ios-sim debug)",
+      "detail": "Build for the iOS simulator (-O0 + .dSYM), install, and launch paused for the debugger.",
+      "type": "shell",
+      "command": "${workspaceFolder}/.vscode/ios-sim-debug.sh",
+      "options": { "cwd": "${workspaceFolder}" },
+      "group": "build",
+      "problemMatcher": []
+    }
+  ]
+}