Fix max_decode_depth to control decoder hierarchy and port generation (#18)

* Initial plan

* Fix max_decode_depth to properly control decoder hierarchy and port generation

Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

* Fix test that relied on old depth behavior

Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

* Update documentation for max_decode_depth parameter

Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

* fix format

* Add variable_depth RDL file and smoke tests for max_decode_depth parameter

Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

* Add variable depth tests for APB3 and AXI4-Lite CPUIFs

Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

* fix

* fix

* bump

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: arnavsacheti <36746504+arnavsacheti@users.noreply.github.com>

src/peakrdl_busdecoder/decode_logic_gen.py

@@ -85,6 +85,20 @@ class DecodeLogicGenerator(BusDecoderListener):
     def enter_AddressableComponent(self, node: AddressableNode) -> WalkerAction | None:
         action = super().enter_AddressableComponent(node)
 
+        should_decode = action == WalkerAction.SkipDescendants
+
+        if not should_decode and self._ds.max_decode_depth == 0:
+            # When decoding all levels, treat leaf registers as decode boundary
+            for child in node.children():
+                if isinstance(child, AddressableNode):
+                    break
+            else:
+                should_decode = True
+
+        # Only generate select logic if we're at the decode boundary
+        if not should_decode:
+            return action
+
         conditions: list[str] = []
         conditions.extend(self.cpuif_addr_predicate(node))
         conditions.extend(self.cpuif_prot_predicate(node))
@@ -146,6 +160,8 @@ class DecodeLogicGenerator(BusDecoderListener):
     def __str__(self) -> str:
         body = self._decode_stack[-1]
         if isinstance(body, IfBody):
+            if len(body) == 0:
+                return f"{self._flavor.cpuif_select}.cpuif_err = 1'b1;"
             with body.cm(...) as b:
                 b += f"{self._flavor.cpuif_select}.cpuif_err = 1'b1;"