ci(fpga): PR-M.3 — wire T-6 drift cosim into regression + CI deps

Adds the T-6 independent reference drift cosim (PR-M.1, c30be89) as a
gated regression check so any future hand-edit drift in NCO_SINE_LUT,
fft_twiddle_*.mem, or DOPPLER_WINDOW_COEFF surfaces on every run.

run_regression.sh: new "Independent Reference Drift (T-6)" check after
the RX-B autocorrelation block in Phase 3. Plain `python3` (no path
sniffing). Distinguishes three states from the script's exit code +
markers:
  rc=0,  PASS markers -> PASS (counts toward `passed`)
  rc=2,  no markers   -> SKIP (counts toward `skipped`)
  rc!=0, FAIL markers -> FAIL (gates the regression)

compare_independent.py: detects missing numpy/scipy at startup and exits
with code 2 plus a [SKIP] marker pointing at `uv sync --group dev`.
Without that, an environment without scipy crashed mid-script and the
regression captured a partial 3-of-13 PASS count.

pyproject.toml: scipy>=1.13 added to the dev dependency group (used by
fpga_reference.doppler_window_ideal() for analytical Cheby ground truth).

.github/workflows/ci-tests.yml: fpga-regression now installs Python
3.12, sets up uv, runs `uv sync --group dev`, and activates the
resulting .venv before bash run_regression.sh. Without the activate
line the runner's system python3 (no scipy) would resolve first and
the drift check would [SKIP] in CI.

Verified locally:
  with venv:    Drift PASS (13 checks), Tests: 43 passed / 0 / 0
  no scipy:     Drift SKIP (msg points at install cmd), 42p / 0f / 1s

9_Firmware/9_2_FPGA/run_regression.sh

@@ -708,6 +708,52 @@ run_test --timeout=600 "RX-B Full-Chain Autocorrelation (tb_rxb_fullchain_latenc
     fft_engine_axi_bridge.v frequency_matched_filter.v \
     chirp_reference_rom.v
 
+# ---------------------------------------------------------------------------
+# T-6 independent reference drift cosim (PR-M).
+# Bytewise spot-checks of NCO_SINE_LUT, fft_twiddle_{16,2048}.mem, and
+# DOPPLER_WINDOW_COEFF against analytical Q15 values, plus end-to-end peak
+# and roundtrip invariants for NCO / FFT / MF / Doppler. Catches the bug
+# class where a transcription error exists identically in both fpga_model.py
+# (RTL-mirroring twin) and the RTL — which the bit-exact cosim cannot detect
+# because both sides of that comparison are computing the same wrong values.
+# Pure Python (numpy + scipy), ~1 s wall, no iverilog compile.
+#
+# Required deps: numpy, scipy (declared in pyproject.toml dev group).
+# Install with:  uv sync --group dev    (from repo root)
+# CI handles this in the fpga-regression job; locally activate the
+# resulting .venv (or use `uv run bash run_regression.sh`).
+# If a dep is missing the script emits a [SKIP] marker and exits with
+# code 2; the regression treats it as SKIP rather than FAIL so the
+# missing-dep state is visible without breaking the gate.
+# ---------------------------------------------------------------------------
+printf "  %-46s" "Independent Reference Drift (T-6)"
+set +e
+drift_output=$(python3 tb/cosim/compare_independent.py 2>&1)
+drift_rc=$?
+set -e
+drift_pass=$(echo "$drift_output" | grep -Ec '^[[:space:]]*\[PASS\]' || true)
+drift_fail=$(echo "$drift_output" | grep -Ec '^[[:space:]]*\[FAIL\]' || true)
+if [[ "$drift_rc" -eq 2 ]]; then
+    # Script signalled missing-dep skip. Show its message body so the
+    # operator knows which package to install.
+    echo -e "${YELLOW}SKIP${NC} (missing python dep — see below)"
+    echo "$drift_output" | sed 's/^/    /'
+    SKIP=$((SKIP + 1))
+elif [[ "$drift_fail" -gt 0 ]]; then
+    echo -e "${RED}FAIL${NC} (pass=$drift_pass, fail=$drift_fail)"
+    ERRORS="$ERRORS\n  Independent Reference Drift: $drift_fail failure(s)"
+    echo "$drift_output" | sed 's/^/    /'
+    FAIL=$((FAIL + 1))
+elif [[ "$drift_pass" -gt 0 && "$drift_rc" -eq 0 ]]; then
+    echo -e "${GREEN}PASS${NC} ($drift_pass checks)"
+    PASS=$((PASS + 1))
+else
+    echo -e "${RED}FAIL${NC} (rc=$drift_rc, no clean PASS/FAIL markers)"
+    ERRORS="$ERRORS\n  Independent Reference Drift: rc=$drift_rc"
+    echo "$drift_output" | sed 's/^/    /'
+    FAIL=$((FAIL + 1))
+fi
+
 echo ""
 
 # ===========================================================================

9_Firmware/9_2_FPGA/tb/cosim/compare_independent.py

@@ -35,7 +35,28 @@ import math
 import sys
 from pathlib import Path
 
-import numpy as np
+# Required: numpy + scipy. If either is missing, exit code 2 with a [SKIP]
+# marker so the regression can distinguish missing-deps from real failures
+# (see run_regression.sh "Independent Reference Drift (T-6)" block).
+_MISSING = []
+try:
+    import numpy as np  # noqa: F401
+except ImportError:
+    _MISSING.append("numpy")
+try:
+    import scipy.signal.windows  # noqa: F401
+except ImportError:
+    _MISSING.append("scipy")
+if _MISSING:
+    print(
+        "  [SKIP] T-6 drift cosim requires Python packages: "
+        f"{', '.join(_MISSING)}.\n"
+        "         Install with: uv sync --group dev   (from repo root)\n"
+        "         or:           pip install numpy scipy"
+    )
+    sys.exit(2)
+
+import numpy as np  # re-import to get module binding now that we know it's there
 
 # Make local imports work when invoked from anywhere
 THIS_DIR = Path(__file__).resolve().parent