FlowDrop 2.0.0-beta1

The 2.x line moves from alpha to beta. No new features: this release exists
because the module did not run on PHP 8.3 -- the floor Drupal 11.3 sets and the
version drupal.org CI tests.

Fixed: 89 promoted constructor properties across 50 classes were declared
readonly beneath Drupal base classes that use DependencySerializationTrait,
whose __wakeup() reassigns them on unserialize. PHP below 8.4 forbids
initializing a readonly property declared in a child class from the parent's
scope, so every such reassignment was a fatal Error. WorkflowExecutionQueueWorker
was the sharpest case: Drupal serializes queue items, so fire-and-forget
workflow execution could not survive the queue. Immutability on value objects
is untouched -- 933 of 1022 readonly declarations are unchanged.

Also fixed: five StateGraph port-resolution tests that had kept the phpunit job
red since before the alpha15 tag.

Changed: the declared PHP floor is 8.3 in composer.json and
flowdrop_chat.info.yml. A no-op for existing sites -- any Drupal 11.3 site
already satisfies it.

Beta, not a release candidate: the @api/@internal boundary that bc-policy.md
requires on every class is unfinished, so the public surface is not yet frozen.
Treat anything undeclared as internal until it is annotated.

Nothing to run on upgrade: no schema updates, no config changes, no behaviour
changes to workflows.