fix: push polled values to HomeKit instead of discarding them

[JanzenMark]

Fixes #1409

The OpenAPI poll fetched each device and discarded the result, so HomeKit was
only ever told a value when a controller read one. The Home app shows the last
value it was notified of, so a stale reading never corrected itself.

What changed

• src/SwitchBotHAPPlatform.ts — record each readable characteristic as it is
  registered, then push new values after every per-device and batched poll.
  Push once immediately after the accessories are registered as well, so the
  Home app shows a value without waiting for the first poll interval.

Two supporting changes make that safe and cheap:

• getState() caches a fetched state briefly and shares one in-flight fetch.
  HomeKit reads every characteristic of an accessory together, so without this
  a push of a seven characteristic accessory costs seven API requests, and
  Homebridge warns that the read handler is slow.
• A device that could not be read is marked, and a marked state pushes nothing.
  The getters report a fallback for a missing field, so pushing one would
  replace a good value in HomeKit with something that looks like a measurement,
  and nothing would correct it. A failed fetch is not cached either, or one
  transient error would hold every characteristic at its fallback for the whole
  window.

A genuine zero is still pushed. Only an unknown value is skipped.

Note on the test change

test/device/getstate-returns-status.spec.ts asserted the exact shape of the
minimal fallback, which now carries that marker, so two assertions there are
updated. That file came from #1404.

Tests

• test/hap-push-updates.spec.ts — known values reach HomeKit, an unreadable
  device pushes nothing, a genuine zero is still pushed, an unknown value is
  skipped, one failing getter does not abandon the rest, the device state is
  read once for the whole accessory, and priming pushes each device once.
• test/device/getstate-cache.spec.ts — a burst of concurrent reads costs one
  fetch, the value is reused inside the window and refetched after it, and a
  failure is marked and not cached.

Full suite, lint, typecheck and build pass.

Scope

Three linked changes rather than one. I kept them together because pushing
without the other two reintroduces the failure this is meant to fix: a
transient read error overwrites good values in HomeKit with zeros that look
real. Happy to split them if you would prefer.