The previous recycle logic waited for all refcounts to hit 0 before
recycling, which never happened under sustained concurrent load (20+
crawls always had at least one active).
New approach:
- Add _browser_version to config signature — bump it to force new contexts
- When threshold is hit: bump version, move old sigs to _pending_cleanup
- New requests get new contexts automatically (different signature)
- Old contexts drain naturally and get cleaned up when refcount hits 0
- Safety cap: max 3 pending browsers draining at once
This means recycling now works under any load pattern — no blocking,
no waiting for quiet moments. Old and new browsers coexist briefly
during transitions.
Includes 12 new tests covering version bumps, concurrent recycling,
safety cap, and edge cases.
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