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/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.