gephi / graphstore / #583

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Add a dedicated monitor for graph observer list access (#298)

* Add a dedicated monitor for graph observer list access

The observers list was previously only safe because every mutation went
through the graph write lock; this adds synchronized(observers) at each
access point so it also stays correct if auto-locking is disabled, and
marks GraphObserverImpl.destroyed volatile for cross-thread visibility.
Adds concurrency tests covering observer create/destroy racing against
version-overflow reset and view destruction.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* Strengthen observer concurrency tests

An independent review found the existing concurrency tests pass even
without the observers-monitor fix, since the default auto-locking
config already serializes every access via the graph write lock — the
monitor only matters when auto-locking is disabled, which nothing
exercised. Adds a test under that config, verified to fail
(ArrayIndexOutOfBoundsException) on the pre-fix code and pass on it.
Also exercises edge-version overflow (previously only node version was
covered) and synchronizes a leftover unguarded list read.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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