feat(llvm): Add optional latch→header connection in LoopForm

- Added NYASH_LOOPFORM_LATCH2HEADER environment variable
- When enabled, latch block jumps back to header (completing the loop)
- When disabled (default), latch remains unreachable (safe mode)
- Preserves header predecessor count stability in default mode

This allows gradual testing of full LoopForm loop structure.

src/backend/llvm/compiler/codegen/instructions/loopform.rs

@@ -134,9 +134,17 @@ pub fn lower_while_loopform<'ctx, 'b>(
     registry.insert(header_bid, (lf.dispatch, tag_phi, payload_phi, lf.latch));
     body_to_header.insert(body_bb, header_bid);
 
-    // Latch: keep unreachable for now (avoid adding a new predecessor to header)
+    // Latch: optionally jump back to header (gated), otherwise keep unreachable to avoid header pred増
     codegen.builder.position_at_end(lf.latch);
+    if std::env::var("NYASH_LOOPFORM_LATCH2HEADER").ok().as_deref() == Some("1") {
+        codegen
+            .builder
+            .build_unconditional_branch(header_llbb)
+            .map_err(|e| e.to_string())
+            .unwrap();
+    } else {
         let _ = codegen.builder.build_unreachable();
+    }
     // Exit: to original after
     codegen.builder.position_at_end(lf.exit);
     codegen

src/backend/llvm/compiler/codegen/mod.rs

@@ -500,6 +500,9 @@ impl LLVMCompiler {
                             }
                         }
                     }
+                    // Note: LoopForm latch→header adds a new LLVM pred not represented in MIR.
+                    // Header PHI normalization for this extra pred will be implemented later
+                    // using a LoopForm-aware finalize that does not rely on MIR inputs.
                 }
             }
         // Finalize function: ensure every basic block is closed with a terminator.