feat: droid management (switch/manage droids) (PR 7/9)

[thePunderWoman]

Summary

PR 7 of the firmware rewrite (PR 1: #35, PR 2: #36, PR 3: #38, PR 4: #39, PR 5: #40, PR 6: #41). Adds Switch Droid and Manage Droids — the two menu items deferred out of PR 6 pending this subsystem, which completes the menu tree (only Display Config, PR 9, remains outside it).

• DroidStore (include/droid_store.h / src/droid_store.cpp): pure in-memory name+PAN ID list, add/remove, capacity-bounded (8 entries). DroidPersistence (src/droid_persistence.cpp) is the thin NVS adapter, joining calibration_store.cpp in the native build/coverage exclusion list.
• DroidSwitcher + XbeeTransport (include/xbee_control.h / src/xbee_control.cpp): pure orchestration of the leave/set-PAN/rejoin sequence, tested against a fake transport. XbeeControl is an honest stub reporting failure at every step — the real XBee SPI/AT-command implementation is PR 8's job. It's not added to the hardware-exclusion list yet since it has no real hardware calls to exclude — it'll join once PR 8 gives it real SPI calls.
• Menu additions, both riding PR 6's framework:
  • Switch Droid — list + result screen, invoking DroidSwitcher (currently always reports "No XBee link" until PR 8 lands).
  • Manage Droids — list with "+ Add New", add flow using TextEntryWidget for name then PAN ID (alphanumeric then hex), delete with confirm. Edit is deliberately out of scope — delete + re-add covers it for now; a straight edit flow can follow later if wanted.
• Factory Reset now also clears the in-memory droid list and reports it through the same consumeDroidStoreChanged() signal a normal add/delete uses, so SnipsController.ino persists it identically either way.

Test plan

• pio test -e native — all 139 test cases pass
• gcovr --exclude 'src/SnipsController\.ino' --exclude 'src/oled\.cpp' --exclude 'src/rgb_led\.cpp' --exclude 'src/calibration_store\.cpp' --exclude 'src/droid_persistence\.cpp' --fail-under-line 90 — 98.3% line coverage (menu.cpp itself at 97%; remaining gaps are defensive branches unreachable through the public state machine)
• pio run -e esp32s3 — builds successfully
• Flash to real hardware once boards arrive: add a droid, verify it persists across reboot, delete it, confirm Factory Reset clears the list, confirm Switch Droid reports "No XBee link" (expected until PR 8)

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