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

apps/tests/macro/if/assign_both_branches.nyash

@@ -0,0 +1,8 @@
+local x = 0
+if (1 < 2) {
+  x = 10
+} else {
+  x = 20
+}
+print(x)
+

docs/guides/scope-hints.md

@@ -26,3 +26,15 @@ Next
 - Wire ScopeEnter/Leave in MirBuilder for function entry/exit and block constructs.
 - Add a simple debug dump when NYASH_MIR_TRACE_HINTS=1.
 
+Runtime Trace
+- Enable: `NYASH_MIR_TRACE_HINTS=1`
+- Backend: any path that builds MIR (e.g., `--backend vm`)
+- Output (stderr):
+  - `[mir][hint] ScopeEnter(0)` at function entry
+  - `[mir][hint] JoinResult(x)` when both branches assign same variable `x`
+  - `[mir][hint] ScopeLeave(0)` at function exit
+
+Example
+- `apps/tests/macro/if/assign_both_branches.nyash` emits JoinResult(x):
+  - `if (cond) { x = 10 } else { x = 20 }`
+  - Both branches assign `x`, builder hints the join.

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"
+