Add cocotb testbench for validating generated bus decoder RTL across APB3, APB4, and AXI4-Lite interfaces (#9)

* Initial plan

* Add cocotb test infrastructure and testbenches for APB3, APB4, and AXI4-Lite

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

* Add integration tests, examples, and documentation for cocotb testbenches

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

* Address code review feedback: use relative imports and update installation docs

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

* Add implementation summary document

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

* Merge cocotb dependencies into test group

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

* Add optional cocotb simulation workflow with Icarus Verilog

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>

tests/cocotb/common/apb4_master.py

@@ -0,0 +1,123 @@
+"""APB4 Master Bus Functional Model for cocotb."""
+
+import cocotb
+from cocotb.triggers import RisingEdge, Timer
+
+
+class APB4Master:
+    """APB4 Master Bus Functional Model."""
+
+    def __init__(self, dut, name, clock):
+        """
+        Initialize APB4 Master.
+
+        Args:
+            dut: The device under test
+            name: Signal name prefix (e.g., 's_apb')
+            clock: Clock signal to use for synchronization
+        """
+        self.dut = dut
+        self.clock = clock
+        self.name = name
+
+        # Get signals
+        self.psel = getattr(dut, f"{name}_PSEL")
+        self.penable = getattr(dut, f"{name}_PENABLE")
+        self.pwrite = getattr(dut, f"{name}_PWRITE")
+        self.paddr = getattr(dut, f"{name}_PADDR")
+        self.pwdata = getattr(dut, f"{name}_PWDATA")
+        self.pstrb = getattr(dut, f"{name}_PSTRB")
+        self.pprot = getattr(dut, f"{name}_PPROT")
+        self.prdata = getattr(dut, f"{name}_PRDATA")
+        self.pready = getattr(dut, f"{name}_PREADY")
+        self.pslverr = getattr(dut, f"{name}_PSLVERR")
+
+    def reset(self):
+        """Reset the bus to idle state."""
+        self.psel.value = 0
+        self.penable.value = 0
+        self.pwrite.value = 0
+        self.paddr.value = 0
+        self.pwdata.value = 0
+        self.pstrb.value = 0
+        self.pprot.value = 0
+
+    async def write(self, addr, data, strb=None):
+        """
+        Perform APB4 write transaction.
+
+        Args:
+            addr: Address to write to
+            data: Data to write
+            strb: Byte strobe mask (default: all bytes enabled)
+
+        Returns:
+            True if write succeeded, False if error
+        """
+        # Calculate strobe if not provided
+        if strb is None:
+            data_width_bytes = len(self.pwdata) // 8
+            strb = (1 << data_width_bytes) - 1
+
+        # Setup phase
+        await RisingEdge(self.clock)
+        self.psel.value = 1
+        self.penable.value = 0
+        self.pwrite.value = 1
+        self.paddr.value = addr
+        self.pwdata.value = data
+        self.pstrb.value = strb
+        self.pprot.value = 0
+
+        # Access phase
+        await RisingEdge(self.clock)
+        self.penable.value = 1
+
+        # Wait for ready
+        while True:
+            await RisingEdge(self.clock)
+            if self.pready.value == 1:
+                error = self.pslverr.value == 1
+                break
+
+        # Return to idle
+        self.psel.value = 0
+        self.penable.value = 0
+
+        return not error
+
+    async def read(self, addr):
+        """
+        Perform APB4 read transaction.
+
+        Args:
+            addr: Address to read from
+
+        Returns:
+            Tuple of (data, error) where error is True if read failed
+        """
+        # Setup phase
+        await RisingEdge(self.clock)
+        self.psel.value = 1
+        self.penable.value = 0
+        self.pwrite.value = 0
+        self.paddr.value = addr
+        self.pprot.value = 0
+
+        # Access phase
+        await RisingEdge(self.clock)
+        self.penable.value = 1
+
+        # Wait for ready
+        while True:
+            await RisingEdge(self.clock)
+            if self.pready.value == 1:
+                data = self.prdata.value.integer
+                error = self.pslverr.value == 1
+                break
+
+        # Return to idle
+        self.psel.value = 0
+        self.penable.value = 0
+
+        return data, error