feat(joinir): Phase 41-4.1 Add NestedIfMerge JoinInst variant

Add structural support for nested if patterns with PHI-sensitive variables:

1. JoinInst::NestedIfMerge in mod.rs
   - conds: Vec<VarId> (outer to inner conditions)
   - merges: Vec<MergePair> (variable updates at deepest level)
   - k_next: Option<JoinContId> (continuation after merge)

2. JSON serialization in json.rs
   - Type: "nested_if_merge"
   - Fields: conds[], merges[], k_next

3. Runner/Bridge stubs
   - JoinIR Runner: Returns error (use VM Bridge instead)
   - VM Bridge: panic placeholder (41-4.3 will implement)

Target: ParserControlBox.parse_loop() (4-level nested if)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/json.rs

@@ -180,6 +180,38 @@ fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
             write!(out, "]")?;
             write!(out, "}}")?;
         }
+        // Phase 41-4: NestedIfMerge instruction JSON serialization
+        JoinInst::NestedIfMerge { conds, merges, k_next } => {
+            write!(out, "{{\"type\":\"nested_if_merge\"")?;
+            // conds array
+            write!(out, ",\"conds\":[")?;
+            for (i, cond) in conds.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{}", cond.0)?;
+            }
+            write!(out, "]")?;
+            // merges array
+            write!(out, ",\"merges\":[")?;
+            for (i, merge) in merges.iter().enumerate() {
+                if i > 0 {
+                    write!(out, ",")?;
+                }
+                write!(out, "{{")?;
+                write!(out, "\"dst\":{}", merge.dst.0)?;
+                write!(out, ",\"then_val\":{}", merge.then_val.0)?;
+                write!(out, ",\"else_val\":{}", merge.else_val.0)?;
+                write!(out, "}}")?;
+            }
+            write!(out, "]")?;
+            // k_next
+            match k_next {
+                Some(k) => write!(out, ",\"k_next\":{}", k.0)?,
+                None => write!(out, ",\"k_next\":null")?,
+            }
+            write!(out, "}}")?;
+        }
         JoinInst::Compute(mir_like) => {
             write!(out, "{{\"type\":\"compute\",\"op\":")?;
             write_mir_like_inst(mir_like, out)?;