feat: Add v0.3 OTA updates — dual partition, esp-ota tool, rollback

Dual OTA partition table (ota_0/ota_1, 1920 KB each) on 4MB flash.
Firmware gains OTA command, LED_OTA double-blink, version in STATUS,
and automatic rollback validation. Pi-side esp-ota tool serves firmware
via HTTP and orchestrates the update flow. esp-fleet gains ota subcommand.

tools/esp-fleet

@@ -2,7 +2,9 @@
 """Query all ESP32 CSI sensors in parallel."""
 
 import concurrent.futures
+import os
 import socket
+import subprocess
 import sys
 
 DEFAULT_PORT = 5501
@@ -14,6 +16,8 @@ SENSORS = [
     ("hollow-acorn", "hollow-acorn.local"),
 ]
 
+ESP_OTA = os.path.join(os.path.dirname(os.path.abspath(__file__)), "esp-ota")
+
 USAGE = """\
 Usage: esp-fleet <command> [args...]
 
@@ -25,11 +29,14 @@ Commands:
   rate <10-100>       Set ping rate on all devices
   power <2-20>        Set TX power on all devices
   reboot              Reboot all devices
+  ota [firmware.bin]   OTA update all devices (sequentially)
 
 Examples:
   esp-fleet status
   esp-fleet identify
-  esp-fleet rate 50"""
+  esp-fleet rate 50
+  esp-fleet ota
+  esp-fleet ota /path/to/firmware.bin"""
 
 
 def query(name, host, cmd):
@@ -54,11 +61,33 @@ def query(name, host, cmd):
         sock.close()
 
 
+def run_ota(firmware=None):
+    """Run OTA on each sensor sequentially."""
+    for name, host in SENSORS:
+        print(f"\n{'='*40}")
+        print(f"OTA: {name} ({host})")
+        print(f"{'='*40}")
+        cmd = [ESP_OTA, host]
+        if firmware:
+            cmd += ["-f", firmware]
+        result = subprocess.run(cmd)
+        if result.returncode != 0:
+            print(f"ERR: OTA failed for {name}, stopping fleet OTA", file=sys.stderr)
+            sys.exit(1)
+    print(f"\nAll {len(SENSORS)} devices updated.")
+
+
 def main():
     if len(sys.argv) < 2 or sys.argv[1] in ("-h", "--help"):
         print(USAGE)
         sys.exit(0 if sys.argv[1:] and sys.argv[1] in ("-h", "--help") else 2)
 
+    # Handle OTA subcommand separately (sequential, not parallel)
+    if sys.argv[1].lower() == "ota":
+        firmware = sys.argv[2] if len(sys.argv) > 2 else None
+        run_ota(firmware)
+        return
+
     cmd = " ".join(sys.argv[1:]).strip().upper()
 
     with concurrent.futures.ThreadPoolExecutor(max_workers=len(SENSORS)) as pool: