docs(phase260): checkpoint P0/P0.1 + update quick first fail

src/mir/join_ir_vm_bridge/joinir_block_converter.rs

@@ -427,9 +427,10 @@ impl JoinIrBlockConverter {
                     effects: EffectMask::PURE,
                 });
 
-                // Phase 131 P2: Preserve tail-call args as jump_args metadata (SSOT for exit wiring)
+                // Phase 131 P2: Preserve tail-call args as legacy jump-args metadata (for exit wiring)
                 //
-                // Merge/ExitArgsCollector uses BasicBlock.jump_args to recover carrier/env values.
+                // The merge pipeline recovers carrier/env values from the legacy jump-args path
+                // (via BasicBlock APIs) when converting Return → Jump to the exit block.
                 // Without this, tail-call blocks look like "no args", forcing fallbacks that can
                 // produce undefined ValueIds in DirectValue mode.
                 if let Some(block) = mir_func.blocks.get_mut(&self.current_block_id) {