chore(tests): retire v1 cross-layer iverilog cosim tier

The v1-era tb_cross_layer_ft2232h.v cosim TB no longer matches
production after the protocol-v2 / opcode dispatch rework (PR-G).
Equivalent v2 coverage now lives in the FPGA regression's
tb_usb_protocol_v2.v and tb_system_opcodes.v.

Removed:
  - tb_cross_layer_ft2232h.v (716 lines)
  - Tier 2 (Verilog cosimulation) from test_cross_layer_contract.py
  - iverilog/vvp tool detection and CI install step in ci-tests.yml

Tier 1 (static parser) and Tier 3 (C stub execution) remain. CI
no longer needs apt-get install iverilog.

contract_parser.py updated to reflect the slimmer two-tier model.

.github/workflows/ci-tests.yml

@@ -106,7 +106,10 @@ jobs:
   # ===========================================================================
   # Cross-Layer Contract Tests (Python ↔ Verilog ↔ C)
   # Validates opcode maps, bit widths, packet layouts, and round-trip
-  # correctness across FPGA RTL, Python GUI, and STM32 firmware.
+  # correctness across FPGA RTL, Python GUI, and STM32 firmware. Runs
+  # entirely as Python static parsers + the Tier 3 C stub; the v1-era
+  # iverilog cosim TB was retired post-PR-G (equivalent v2 coverage now
+  # lives in the FPGA regression's tb_usb_protocol_v2 / tb_system_opcodes).
   # ===========================================================================
   cross-layer-tests:
     name: Cross-Layer Contract Tests
@@ -124,9 +127,6 @@ jobs:
       - name: Install dependencies
         run: uv sync --group dev
 
-      - name: Install Icarus Verilog
-        run: sudo apt-get update && sudo apt-get install -y iverilog
-
       - name: Run cross-layer contract tests
         run: >
           uv run pytest