Use the node's admin password for the migration bridge AP

The Wi-Fi, LoRa first-stage and Partition Expander bridges serve an
unauthenticated /update behind an AP whose password was fixed and published,
so anyone in range during a migration could upload firmware while the private
key is staged. Use the old node's saved admin password as the WPA2
passphrase, read from /com_prefs or /node_prefs at offset 56 as the firmware
loads it. The operator already used that password to start the old updater.

Fall back to the published password, with a serial warning, when the saved
admin password is missing, unreadable, shorter than 8 characters or not
printable ASCII, so no node becomes unreachable. The legacy filesystem is
mounted without formatting and the password is never printed. Migration
admission, flash sequencing, /update gating and the USB bridge are unchanged.
Update both guides and the package README.

Add a sanitizer-backed host test that compiles the bridge's actual Wi-Fi
block against fake SPIFFS and WiFi: both boots, file precedence, WPA2 length
and terminator bounds, printable characters, stock 1.10/1.11 file sizes,
truncated fields, and mount, seek and partial-read failures. It fails 8 of 8
on the previous code and catches 27 of 27 code mutations. Verified 13 related
host test files and the esp32_s3_4mb_partition_migrator build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/package_esp32_partition_migration.py

@@ -506,8 +506,12 @@ installing the bridge; the bridge does not itself repeat normal traffic.
 1. On the old node, run `start ota ap` through its authenticated terminal.
    Join its `MeshCore-OTA` Wi-Fi and upload `wifi-bridge.bin` at the exact
    `Started:` URL returned by the command (the port can vary by old build).
-2. Keep power stable. Join `MeshCore-Migrate` (password `meshcore-migrate`),
-   open `http://192.168.4.1/`, and wait for **Expanded layout ready**.
+2. Keep power stable. Join `MeshCore-Migrate` using the node's saved admin
+   password as the Wi-Fi password, open `http://192.168.4.1/`, and wait for
+   **Expanded layout ready**. If that password is shorter than 8 characters
+   or was never saved, the bridge falls back to the published
+   `meshcore-migrate`, which anyone nearby can use; set a longer admin
+   password before step 1.
 3. At that page's `/update`, upload `full-application.bin`.
 """
     usb_instructions = f"""## Host-managed USB route