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/ios-sim-debug.sh

@@ -0,0 +1,29 @@
+#!/usr/bin/env bash
+# preLaunchTask for "Debug SxChess (iOS sim)": build SxChess for the iOS
+# simulator with debug info, install it to a single simulator, and launch it
+# PAUSED for a debugger (--wait-for-debugger). The launch.json `custom` config
+# then `target create`s the local binary (so the .dSYM resolves breakpoints)
+# and `process attach -n SxChess`es the waiting process.
+set -e
+
+SX="/Users/agra/projects/sx/zig-out/bin/sx"
+DSYMUTIL="/opt/homebrew/opt/llvm@19/bin/dsymutil"
+# Reuse ONE simulator (sx-test-ios18). Swap the UDID for a different device.
+SIM="E8DF755C-997D-49D7-9DB0-CFA48F8254CD"
+APP="sx-out/ios/M3te.app"
+BUNDLE="co.swipelab.m3te"
+
+# Debug build: -O0 + DWARF (--emit-obj keeps the object), then a portable .dSYM.
+"$SX" build --target ios-sim --emit-obj main.sx
+"$DSYMUTIL" "$APP/M3te"
+
+# Boot the sim (idempotent) and surface the Simulator UI.
+xcrun simctl boot "$SIM" 2>/dev/null || true
+xcrun simctl bootstatus "$SIM" >/dev/null 2>&1 || true
+open -a Simulator
+
+xcrun simctl install "$SIM" "$APP"
+xcrun simctl terminate "$SIM" "$BUNDLE" >/dev/null 2>&1 || true
+
+# Launch paused; the app waits at its entry until lldb attaches and continues.
+xcrun simctl launch --wait-for-debugger "$SIM" "$BUNDLE"

.vscode/launch.json

@@ -0,0 +1,27 @@
+{
+  "version": "0.2.0",
+  "configurations": [
+    {
+      "type": "lldb",
+      "request": "launch",
+      "name": "Debug M3te (macOS)",
+      "program": "${workspaceFolder}/sx-out/macos/M3te.app/Contents/MacOS/M3te",
+      "cwd": "${workspaceFolder}",
+      "preLaunchTask": "sx build (macos debug)"
+    },
+    {
+      "type": "lldb",
+      "request": "custom",
+      "name": "Debug M3te (iOS sim)",
+      "preLaunchTask": "sx build+launch (ios-sim debug)",
+      "targetCreateCommands": [
+        "target create '${workspaceFolder}/sx-out/ios/M3te.app/M3te'"
+      ],
+      "processCreateCommands": [
+        "process attach -n M3te",
+        "process handle SIGSTOP --notify false --stop false --pass false",
+        "process continue"
+      ]
+    }
+  ]
+}

.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": []
+    }
+  ]
+}