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Closes #3482

The dirty module (which uses asyncio and concurrent.futures) was being
imported at gunicorn startup via gunicorn.arbiter. This caused
concurrent.futures to be imported before user code could call
gevent.monkey.patch_all(), breaking gevent's monkey-patching.

Changes:
- gunicorn/arbiter.py: Import DirtyArbiter and set_dirty_socket_path
  lazily inside spawn_dirty_arbiter() instead of at module level
- gunicorn/dirty/worker.py: Import ThreadPoolExecutor lazily inside
  run() method instead of at module level
- Add tests/workers/test_gevent_import_order.py with 5 tests verifying:
  - concurrent.futures is NOT imported when gunicorn.arbiter loads
  - gevent patching works correctly with gunicorn
  - Reproduces the exact scenario from the bug report gist

This ensures gevent's monkey.patch_all() can run before concurrent.futures
is imported, allowing proper patching of threading primitives.

…mport

fix: lazy import dirty module for gevent compatibility

Closes #3484

When a client disconnects during an ASGI request, the worker now:
1. Sends http.disconnect message to the app's receive queue
2. Allows a configurable grace period for cleanup (default: 3 seconds)
3. Only cancels the task after the grace period expires

This follows the ASGI HTTP Connection Scope spec which defines
http.disconnect as the message apps should receive when clients
disconnect: https://asgi.readthedocs.io/en/latest/specs/www.html#disconnect-receive-event

The grace period prevents CancelledError from propagating to async
database operations, allowing SQLAlchemy and other async DB libraries
to properly reset their connection pools.

New config option: --asgi-disconnect-grace-period (default: 3 seconds)

fix: graceful disconnect handling for ASGI worker

- Fix nginx config to use keepalive with upstream (was sending
  Connection: close which caused premature connection closure)
- Add _safe_write() to handle socket errors (EPIPE, ECONNRESET,
  ENOTCONN) gracefully when client disconnects
- Fix ASGI scope server/client to always be 2-tuples for IPv6
  compatibility (IPv6 sockets return 4-tuples)
- Add write_eof() before close() to ensure buffered data is flushed
- Bind to [::] for dual-stack IPv4/IPv6 support in test containers

Consolidate the repeated pattern for normalizing socket addresses
to ASGI-compatible (host, port) tuples into a single utility function.

- Bump version to 25.0.2
- Update copyright year to 2026 in LICENSE and NOTICE
- Add license headers to all Python source files
- Add changelog entry for 25.0.2