Add unit tests for cumulative energy meter offset/rollover logic

[Bre77]

Intent

• Add direct unit coverage for updateCumulativeMeter in lib/TeslemetryDevice.ts, which has no tests today despite three past releases (0.1.2, 0.1.3, 0.2.0) fixing energy-meter/unit-conversion bugs in this area
  • Offset initialization from the existing capability value on the first reading
  • Monotonic accumulation across successive same-day readings
  • Day rollover, including back-to-back rollovers across several days
  • No-op behavior once destroyed is set, including mid-flight after the offset write but before the rest of the update completes
• Test-only change: lib/TeslemetryDevice.ts is untouched

Note for reviewers

Writing these tests surfaced a gap, not fixed here: the day-rollover check only triggers on a dateKey change. A same-day raw reading that comes back lower than the previous one (e.g. a transient API glitch) is not guarded against and passes straight through as a decreasing capability value, which is exactly the kind of regression Homey's cumulative-meter energy tracking assumes can't happen. See the passes a lower same-day raw reading straight through as a decreasing value test, which documents the current behavior rather than asserting a fix. Flagging for a separate PR if that's worth guarding against.