Fix decoder generation for external nested addressable components and add max-decode-depth parameter (#12)

* Initial plan

* Fix bus decoder to skip external nested components

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

* Optimize external children check using generator expressions

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

* Add max-decode-depth CLI argument

Added --max-decode-depth argument that:
- Is added to CLI arguments in __peakrdl__.py
- Piped into design state via ExporterKwargs and DesignStateKwargs
- Used to control max depth in listener.py
- All 66 tests pass including new test for the parameter

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

---------

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/design_state.py

@@ -14,6 +14,7 @@ class DesignStateKwargs(TypedDict, total=False):
     package_name: str
     address_width: int
     cpuif_unroll: bool
+    max_decode_depth: int
 
 
 class DesignState:
@@ -35,6 +36,7 @@ class DesignState:
         user_addr_width: int | None = kwargs.pop("address_width", None)
 
         self.cpuif_unroll: bool = kwargs.pop("cpuif_unroll", False)
+        self.max_decode_depth: int = kwargs.pop("max_decode_depth", 1)
 
         # ------------------------
         # Info about the design