Add multi-speaker tracking with beam steering (#5)

multi_speaker.py: Tracks up to 2 speakers simultaneously. When 2 distinct
DoA angles are detected (30°+ apart) for >1s, locks the XVF3800's fixed
beams onto each speaker. Releases back to auto mode when only 1 speaker
remains (3s timeout). Manages beam gating so only the speaking beam is active.

xvf3800.py: Added beam steering commands — enable_fixed_beams(),
set_beam_azimuths(), enable_beam_gating(), read_all_beams().
Manager gets steer_beams() and release_beams() convenience methods.

headmic.py: Wire multi-speaker tracker into DoA loop. New endpoint:
GET /speakers/tracked — current speaker positions, beam mode, lock state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

xvf3800.py

@@ -40,8 +40,15 @@ LED_EFFECT_CMD    = 12  # 0=off, 1=breath, 2=rainbow, 3=solid, 4=doa, 5=ring
 AEC_RESID = 33
 AEC_AZIMUTH_CMD           = 75   # 4 floats: beam1, beam2, free-running, auto-select (radians)
 AEC_SPENERGY_CMD          = 80   # 4 floats: speech energy per beam (>0 = speech)
+# Beam steering (resid=33)
+AEC_FIXED_BEAMS_ON_CMD     = 37  # rw uint8: 0=off, 1=on — enable fixed beam mode
+AEC_FIXED_BEAMS_AZ_CMD     = 81  # rw 2 floats (radians): beam 1 azimuth, beam 2 azimuth
+AEC_FIXED_BEAMS_ELEV_CMD   = 82  # rw 2 floats (radians): beam 1 elevation, beam 2 elevation
+AEC_FIXED_BEAMS_GATING_CMD = 83  # rw uint8: 0=off, 1=on — silence beams without speech
+
 AUDIO_MGR_RESID           = 35
 AUDIO_MGR_SELECTED_AZ_CMD = 11   # 2 floats: processed DoA, auto-select DoA (radians)
+AUDIO_MGR_SELECTED_CH_CMD = 12   # rw 2 uint8: which beam goes to L/R output channel
 LED_BRIGHTNESS_CMD = 14
 LED_COLOR_CMD     = 16  # single uint32 color (confirmed: xvf_host LED_COLOR cmdid=16)
 LED_DOA_COLOR_CMD = 17  # two uint32 values: base + indicator
@@ -129,6 +136,41 @@ class XVF3800:
 
         return int(angle_deg) % 360, vad
 
+    def read_all_beams(self) -> dict:
+        """Read all 4 beam azimuths: beam1, beam2, free-running, auto-select."""
+        import math
+        data = self._read_float(AEC_RESID, AEC_AZIMUTH_CMD, 4)
+        if len(data) < 17:  # 1 status + 4*4 bytes
+            return {}
+        beams = struct.unpack_from("<ffff", data, 1)
+        return {
+            "beam1_deg": round(math.degrees(beams[0]) % 360, 1),
+            "beam2_deg": round(math.degrees(beams[1]) % 360, 1),
+            "free_running_deg": round(math.degrees(beams[2]) % 360, 1),
+            "auto_select_deg": round(math.degrees(beams[3]) % 360, 1),
+        }
+
+    # --- Beam steering ---
+
+    def enable_fixed_beams(self, on: bool = True):
+        """Enable/disable fixed beam mode. When on, beams lock to set azimuths."""
+        self._write(AEC_RESID, AEC_FIXED_BEAMS_ON_CMD, bytes([1 if on else 0]))
+
+    def set_beam_azimuths(self, beam1_deg: float, beam2_deg: float):
+        """Set fixed beam directions in degrees (0=front, 90=right, 180=back, 270=left)."""
+        import math
+        b1_rad = math.radians(beam1_deg)
+        b2_rad = math.radians(beam2_deg)
+        self._write(AEC_RESID, AEC_FIXED_BEAMS_AZ_CMD, struct.pack("<ff", b1_rad, b2_rad))
+
+    def enable_beam_gating(self, on: bool = True):
+        """Enable/disable beam gating. When on, only the beam with speech is active."""
+        self._write(AEC_RESID, AEC_FIXED_BEAMS_GATING_CMD, bytes([1 if on else 0]))
+
+    def disable_fixed_beams(self):
+        """Return to auto beam mode."""
+        self.enable_fixed_beams(False)
+
     # --- LEDs ---
 
     def led_off(self):
@@ -280,6 +322,20 @@ class XVF3800Manager:
             if dev:
                 dev.led_doa()
 
+    def steer_beams(self, beam1_deg: float, beam2_deg: float):
+        """Steer both arrays' fixed beams to the same directions."""
+        for dev in [self.left, self.right]:
+            if dev:
+                dev.enable_fixed_beams(True)
+                dev.set_beam_azimuths(beam1_deg, beam2_deg)
+                dev.enable_beam_gating(True)
+
+    def release_beams(self):
+        """Return both arrays to auto beam mode."""
+        for dev in [self.left, self.right]:
+            if dev:
+                dev.disable_fixed_beams()
+
     def read_both_doa(self) -> dict:
         """Read DoA from both arrays."""
         result = {}