mir(hints): add JoinResult trace smoke; add assign_both_branches case; docs: trace usage in scope-hints; all smokes pass

tools/test/smoke/mir/hints_join_result_smoke.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+root=$(cd "$(dirname "$0")"/../../../.. && pwd)
+bin="$root/target/release/nyash"
+src="apps/tests/macro/if/assign_both_branches.nyash"
+
+if [ ! -x "$bin" ]; then
+  echo "nyash binary not found at $bin; build first (cargo build --release)" >&2
+  exit 1
+fi
+
+export NYASH_MIR_TRACE_HINTS=1
+
+# Capture stderr; VM backend is sufficient (MIR builder runs)
+out=$({ "$bin" --backend vm "$src" 1>/dev/null; } 2>&1 || true)
+
+echo "$out" | rg -q "\[mir\]\[hint\] JoinResult\(x\)" || {
+  echo "[FAIL] missing JoinResult(x) hint" >&2
+  printf '%s\n' "$out" | tail -n 60 >&2
+  exit 2
+}
+
+echo "[OK] MIR hints JoinResult trace smoke passed"
+