feat: on-device menu core, Device Info, Factory Reset (PR 6/9)

[thePunderWoman]

Summary

PR 6 of the firmware rewrite (PR 1: #35, PR 2: #36, PR 3: #38, PR 4: #39, PR 5: #40). Adds the on-device menu system.

• MenuController (include/menu.h / src/menu.cpp): pure state machine. Knows nothing about real buttons or the display. Held-combo detection (Left Up+Down for 1s) opens the menu from inactive; once open, onUp()/onDown() scroll, onEnter() confirms, onBack() cancels/exits.
• renderMenuScreen(): pure function deciding what text goes on screen for the menu's current state — the existing OledDisplay/ScreenBuffer split from PR 3 does the actual drawing.
• Menu tree covers what's genuinely local and already has everything it needs:
  • Calibrate Stick / Calibrate Trigger — wires up the guided-calibration flows built in PR 5 (previously unused), persisting results via CalibrationStore.
  • Device Info — placeholder for now; querying the XBee's SL needs PR 8's SPI transport, which doesn't exist yet.
  • Factory Reset — confirm-gated, currently clears calibration data; will extend to the droid list once DroidStore exists (PR 7).
  • Switch Droid/Manage Droids (PR 7) and Display Config (PR 9) aren't in the tree yet — they need subsystems that don't exist.
• TextEntryWidget (include/text_entry.h / src/text_entry.cpp): the reusable one-character-at-a-time entry widget (full alphanumeric, or hex-only) PR 7's Manage Droids screen will need for name/PAN ID entry. Built now alongside the rest of the framework per the rewrite plan; not wired to anything yet.
• SnipsController.ino: Left Up/Down + Stick Click + Bumper are "stolen" for nav only while the menu is open — harmless since the packet protocol (PR 8) doesn't exist yet for Amidala to see them any other way. Display only redraws on an actual state change, not every tick.

Test plan

• pio test -e native — all 104 test cases pass
• gcovr --exclude 'src/SnipsController\.ino' --exclude 'src/oled\.cpp' --exclude 'src/rgb_led\.cpp' --exclude 'src/calibration_store\.cpp' --fail-under-line 90 — 98.5% line coverage (remaining gaps are defensive branches unreachable through the public state machine)
• pio run -e esp32s3 — builds successfully
• Flash to real hardware once boards arrive: hold Left Up+Down for 1s to open the menu, scroll through all four items, run both calibration flows end to end, confirm Factory Reset clears calibration, confirm the display restores the boot screen when the menu closes

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