SearchIndex: rewrite Lock as static registry with reference counting

Replace the instance-based Lock class with a static registry that
tracks held locks per-process with reference counting. This sol

SearchIndex: rewrite Lock as static registry with reference counting

Replace the instance-based Lock class with a static registry that
tracks held locks per-process with reference counting. This solves
three problems:

- Split indexes (w3, w4, ...) share a single lock name and now
coordinate naturally via the registry
- Multiple callers can acquire the same lock without conflict
- Indexes enforce their own writability through lock()/unlock()
methods on AbstractIndex

The Lock registry manages both the filesystem lock (mkdir) and the
in-process tracking. The first acquire creates the directory, subsequent
acquires increment the refcount. Release decrements, and only removes
the directory when the count reaches zero.

Note: I am not sure if implementing this as a static object is a great
idea or if we should pass an instance through the collection to the
indexes...

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Working fulltext collection and search

This finalizes the FulltextCollection and FulltextCollectionSearch
classes. Proper locking is implemented, tests have been enhanced.

It should be possible to

Working fulltext collection and search

This finalizes the FulltextCollection and FulltextCollectionSearch
classes. Proper locking is implemented, tests have been enhanced.

It should be possible to reimplement the page full text search on top of
it.

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