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/__peakrdl__.py

@@ -111,6 +111,15 @@ class Exporter(ExporterSubcommandPlugin):
             """,
         )
 
+        arg_group.add_argument(
+            "--max-decode-depth",
+            type=int,
+            default=1,
+            help="""Maximum depth for address decoder to descend into nested
+            addressable components. Default is 1.
+            """,
+        )
+
     def do_export(self, top_node: "AddrmapNode", options: "argparse.Namespace") -> None:
         cpuifs = self.get_cpuifs()
 
@@ -123,4 +132,5 @@ class Exporter(ExporterSubcommandPlugin):
             package_name=options.package_name,
             address_width=options.addr_width,
             cpuif_unroll=options.unroll,
+            max_decode_depth=options.max_decode_depth,
         )