Replace broken slowrx dependency with pure Python SSTV decoder

slowrx is a GTK GUI app that doesn't support CLI usage, so the SSTV
decoder was silently failing. This replaces it with a pure Python
implementation using numpy and Pillow that supports Robot36/72,
Martin1/2, Scottie1/2, and PD120/180 modes via VIS header auto-detection.

Key implementation details:
- Generalized Goertzel (DTFT) for exact-frequency tone detection
- Vectorized batch Goertzel for real-time pixel decoding performance
- Overlapping analysis windows for short-window frequency estimation
- VIS header detection state machine with parity validation
- Per-line sync re-synchronization for drift tolerance

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/sstv/constants.py

@@ -0,0 +1,92 @@
+"""SSTV protocol constants.
+
+VIS (Vertical Interval Signaling) codes, frequency assignments, and timing
+constants for all supported SSTV modes per the SSTV protocol specification.
+"""
+
+from __future__ import annotations
+
+# ---------------------------------------------------------------------------
+# Audio / DSP
+# ---------------------------------------------------------------------------
+SAMPLE_RATE = 48000  # Hz - standard audio sample rate used by rtl_fm
+
+# Window size for Goertzel tone detection (5 ms at 48 kHz = 240 samples)
+GOERTZEL_WINDOW = 240
+
+# Chunk size for reading from rtl_fm (100 ms = 4800 samples)
+STREAM_CHUNK_SAMPLES = 4800
+
+# ---------------------------------------------------------------------------
+# SSTV tone frequencies (Hz)
+# ---------------------------------------------------------------------------
+FREQ_VIS_BIT_1 = 1100     # VIS logic 1
+FREQ_SYNC = 1200           # Horizontal sync pulse
+FREQ_VIS_BIT_0 = 1300      # VIS logic 0
+FREQ_BREAK = 1200          # Break tone in VIS header (same as sync)
+FREQ_LEADER = 1900         # Leader / calibration tone
+FREQ_BLACK = 1500          # Black level
+FREQ_WHITE = 2300          # White level
+
+# Pixel luminance mapping range
+FREQ_PIXEL_LOW = 1500      # 0 luminance
+FREQ_PIXEL_HIGH = 2300     # 255 luminance
+
+# Frequency tolerance for tone detection (Hz)
+FREQ_TOLERANCE = 50
+
+# ---------------------------------------------------------------------------
+# VIS header timing (seconds)
+# ---------------------------------------------------------------------------
+VIS_LEADER_MIN = 0.200     # Minimum leader tone duration
+VIS_LEADER_MAX = 0.500     # Maximum leader tone duration
+VIS_LEADER_NOMINAL = 0.300 # Nominal leader tone duration
+VIS_BREAK_DURATION = 0.010 # Break pulse duration (10 ms)
+VIS_BIT_DURATION = 0.030   # Each VIS data bit (30 ms)
+VIS_START_BIT_DURATION = 0.030  # Start bit (30 ms)
+VIS_STOP_BIT_DURATION = 0.030   # Stop bit (30 ms)
+
+# Timing tolerance for VIS detection
+VIS_TIMING_TOLERANCE = 0.5  # 50% tolerance on durations
+
+# ---------------------------------------------------------------------------
+# VIS code → mode name mapping
+# ---------------------------------------------------------------------------
+VIS_CODES: dict[int, str] = {
+    8:   'Robot36',
+    12:  'Robot72',
+    44:  'Martin1',
+    40:  'Martin2',
+    60:  'Scottie1',
+    56:  'Scottie2',
+    93:  'PD120',
+    95:  'PD180',
+    # Less common but recognized
+    4:   'Robot24',
+    36:  'Martin3',
+    52:  'Scottie3',
+    55:  'ScottieDX',
+    113: 'PD240',
+    96:  'PD90',
+    98:  'PD160',
+}
+
+# Reverse mapping: mode name → VIS code
+MODE_TO_VIS: dict[str, int] = {v: k for k, v in VIS_CODES.items()}
+
+# ---------------------------------------------------------------------------
+# Common SSTV modes list (for UI / status)
+# ---------------------------------------------------------------------------
+SSTV_MODES = [
+    'PD120', 'PD180', 'Martin1', 'Martin2',
+    'Scottie1', 'Scottie2', 'Robot36', 'Robot72',
+]
+
+# ISS SSTV frequency
+ISS_SSTV_FREQ = 145.800  # MHz
+
+# Speed of light in m/s
+SPEED_OF_LIGHT = 299_792_458
+
+# Minimum energy ratio for valid tone detection (vs noise floor)
+MIN_ENERGY_RATIO = 5.0