fix: pad short PAN IDs for the XBee, and force router role at boot

[thePunderWoman]

Summary

• Short PAN IDs now work end to end. A PAN ID like 4133 could be entered, saved and displayed, but switching to it failed with "Set PAN failed" — XbeeControl::setPanId requires exactly 16 hex digits and rejected it before anything reached the radio. The boot-time droid-name lookup also strcmp'd the saved 4133 against the module's 0000000000004133, so the Droid Name complication showed "(none)" after every reboot.
  • New pure PanId::normalize / PanId::equivalent (uppercase, left-pad to 16).
  • DroidSwitcher::switchTo pads before calling the transport; an invalid ID returns the new kInvalidPanId ("Invalid PAN ID") without touching the network.
  • New DroidStore::findByPanId, used by SnipsController.ino at boot in place of strcmp.
  • Finishing PAN ID entry with nothing typed now restarts the entry rather than saving an ID that pads to all zeros (Factory Reset's "unconfigured" value).
• Router role enforced at boot. New ensureRouterRole() reads the XBee's CE; if it isn't 0 it writes CE=0, then WR and AC. Failures are logged to Serial and boot continues.

Notes for review

• The CE → WR → AC sequence in XbeeControl is best-effort against Digi's manual and not yet validated on hardware (same caveat the class already carries). Worth confirming during bring-up that AC is enough for a coordinator→router change to take effect, versus needing a reset.
• CE=0 means "join a network"; whether the module ends up a router or an end device is governed by SM (sleep mode). This PR only checks/sets CE, as requested.
• Stored PAN IDs stay as typed (4133); padding happens only when talking to the radio or comparing.

Test plan

• pio test -e native — 252 passed, including new test_pan_id and test_xbee_role suites and regression tests in test_droid_store, test_droid_switcher and test_menu
• pio run -e esp32s3 builds (covers .ino and xbee_control.cpp, which aren't compiled natively)
• Coverage: 97.9% line coverage locally via gcovr with the CI filters (threshold 90%); all new files at 100%
• On hardware: switch to a droid saved as a short PAN ID; boot with CE=1 and confirm it comes up as a router

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