test(fpga): wire 4 orphan TBs; add nightly DDC fuzz CI job

Regression coverage additions in run_regression.sh:
  - Phase 2: tb_ddc_400m (standalone DDC unit, 7 checks)
  - Phase 3: tb_freq_matched_filter (14 checks)
  - Phase 4: tb_ddc_input_interface (26 checks), tb_latency_buffer
    (13 checks)

All four existed in tb/ but had no regression runner entry; now gate
every push/PR. Deletes tb/tb_multiseg_cosim.v — stale against the
current RP_FFT_SIZE=2048 / RP_LONG_SEGMENTS_3KM=2 (TB hardcoded 1024/4,
15/32 checks fail on current RTL). Re-add when the multi-segment TB
is reworked for the 2048-point pipeline.

CI: new fpga-fuzz job running test_ddc_cosim_fuzz.py -m slow
(100-seed sweep). Gated to schedule (07:00 UTC daily) +
workflow_dispatch so PRs stay fast.

.github/workflows/ci-tests.yml

@@ -5,6 +5,9 @@ on:
     branches: [main, develop]
   push:
     branches: [main, develop]
+  schedule:
+    - cron: "0 7 * * *"   # Nightly 07:00 UTC — slow-path fuzz
+  workflow_dispatch:
 
 jobs:
   # ===========================================================================
@@ -114,3 +117,34 @@ jobs:
           uv run pytest
           9_Firmware/tests/cross_layer/test_cross_layer_contract.py
           -v --tb=short
+
+  # ===========================================================================
+  # DDC Co-Sim Fuzz (slow, nightly + manual dispatch only)
+  # 100 random seeds through the full DDC pipeline vs Python radar_scene model.
+  # Gated on schedule/workflow_dispatch to keep PRs fast; skipped on push/PR.
+  # ===========================================================================
+  fpga-fuzz:
+    name: FPGA DDC Fuzz (slow)
+    runs-on: ubuntu-latest
+    if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - uses: astral-sh/setup-uv@v5
+
+      - 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 DDC fuzz (100-seed sweep)
+        run: >
+          uv run pytest -m slow
+          9_Firmware/tests/cross_layer/test_ddc_cosim_fuzz.py
+          -v --tb=short