refactor(station): ask the producer whether publishing still works

[lxsaah]

Why

StationHandle::is_closed kept its own copy of aimdb's fork generation and compared it against the current one:

self.closed.load(Ordering::Acquire) || aimdb_sync::fork::forked_since(self.made_in)

That needs aimdb_sync::fork to be public, and it is not — aimdb-sync made the module crate-private in aimdb#235. This crate does not currently compile against aimdb main, with three E0603s in weather-station/src/handle.rs.

What

It now asks the producer it publishes through:

self.closed.load(Ordering::Acquire) || self.temperature.check().is_err()

SyncProducer::check() is added upstream in aimdb-dev/aimdb#236 for exactly this. The made_in field goes with it, and the word "fork" survives here only in a sentence explaining what the state means.

Three things improve beyond compiling again

The station no longer knows what a fork is. It asks a question about publishing and gets an answer about publishing. The mechanism stays aimdb's business.

The answer cannot drift. There were two checks that happened to agree — this one, and the one inside set(). Now there is one, used by both.

It catches more. A handle dropped without shutdown() left closed false and the generation current, so is_closed() reported open while every publish failed with RuntimeShutdown. The producer catches that too.

What stays

closed: AtomicBool remains. While detach_timeout is joining, the Arc is briefly alive and the producer would still report usable, so the flag still covers that window.

The lock-ordering argument is unchanged — Weak::upgrade takes no lock, so a getter under the GIL still cannot block behind a shutdown. But three comments describing is_closed as "reads an atomic" would have read as stale to the next person checking that argument, so they now say "takes no lock".

Verification

• weather-station: 16 tests green under both tokio-runtime,rustls,sync and tokio-runtime,sync; clippy clean on both.
• weather-station-cpp and weather-station-py check and clippy clean.
• make spike-cpp all green, including the round that matters:

after fork(), the child has no runtime thread
  ok    the parent keeps publishing across the fork — 2 of the parent's 2 readings arrived
  ok    a forked child is told the station is closed — is_closed() reported closed
  ok    a forked child's publish is refused, not silently dropped — threw
  ok    no phantom reading reached the broker

Merge order

Depends on aimdb-dev/aimdb#236. That lands first — this does not compile without it.

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