fix(tools): reject CSV metacharacters in flash_device.py inputs

device-id, location-id, wifi-ssid, and wifi-password were interpolated
directly into the NVS partition CSV. A value containing comma, double
quote, CR, or LF would split the field/row and silently provision the
wrong NVS keys — easiest concrete failure: a Wi-Fi password containing
a comma. Validate operator-supplied strings before generating the CSV.

Add an empty tools/__init__.py so the regression tests can import the
helper as 'tools.flash_device' (matches the existing 'server.*' test
pattern).

Found via adversarial review (run 2026-05-01-192928, gpt-5.5 reviewer).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/flash_device.py

@@ -28,6 +28,25 @@ NVS_NAMESPACE        = "doorcounter"
 NVS_PARTITION_OFFSET = "0x9000"
 NVS_PARTITION_SIZE   = "0x5000"   # matches firmware partition table (20KB)
 
+# Characters that would change the field/row structure of the NVS-CSV format
+# (key,type,encoding,value). A value containing any of these would either
+# split into more fields or add rows, silently provisioning the wrong keys.
+_CSV_FORBIDDEN = (",", '"', "\n", "\r")
+
+
+def _reject_csv_metacharacters(name, value):
+    """Exit with an error if value contains a character that would corrupt
+    the NVS CSV. Used for operator-supplied strings (device id, location id,
+    WiFi credentials)."""
+    for c in _CSV_FORBIDDEN:
+        if c in value:
+            print(
+                f"Error: --{name} contains forbidden character {c!r}; "
+                f"this would corrupt the NVS partition CSV.",
+                file=sys.stderr,
+            )
+            sys.exit(1)
+
 
 def build_nvs_csv(device_id, location_id, hmac_secret,
                   wifi_ssid=None, wifi_pass=None, line_offset=50):
@@ -78,6 +97,13 @@ def main():
         print("Error: --line-offset must be 0-100", file=sys.stderr)
         sys.exit(1)
 
+    _reject_csv_metacharacters("device-id",   args.device_id)
+    _reject_csv_metacharacters("location-id", args.location_id)
+    if args.wifi_ssid is not None:
+        _reject_csv_metacharacters("wifi-ssid",     args.wifi_ssid)
+    if args.wifi_password is not None:
+        _reject_csv_metacharacters("wifi-password", args.wifi_password)
+
     with tempfile.TemporaryDirectory() as tmp:
         csv_path = os.path.join(tmp, "nvs.csv")
         bin_path = os.path.join(tmp, "nvs.bin")