Fix polling (neon) driver reentrancy panic on in-read filter writes

[Meemaw]

The polling driver's StreamOpsInner::with is Cell::take().unwrap() with no reentrancy guard. The driver calls StreamItem::read (-> filter read processing -> update_read_status) inside a with scope. If a filter produces >= write_buf_threshold (8192) bytes of write data during read processing while writes are paused, the io stream initiates a direct write (Handle::write -> WeakStreamCtl::with), which re-enters with on the same already-taken cell -> None.unwrap() panic, killing the arbiter thread and the connection.

This is the exact failure mode of a TLS-style filter that emits a large handshake/control burst in response to incoming records.

Fix: route direct writes through a new write_stream that, when the streams slab is already borrowed (we are inside event handling), defers the write to check_delayed_writes, draining it once the slab is released

• mirroring the existing delayed_drop mechanism. No more reentrant take.

Regression test test_filter_large_write_during_read_processing exercises the polling backend (the default driver on non-Linux hosts): it panics with "called Option::unwrap() on a None value" at polling/stream.rs without this fix and passes with it.

[codecov]

Codecov Report

❌ Patch coverage is 97.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.27%. Comparing base (61295aa) to head (55afd4a).

Files with missing lines	Patch %	Lines
ntex-net/src/polling/stream.rs	97.43%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #906      +/-   ##
==========================================
+ Coverage   90.25%   90.27%   +0.01%     
==========================================
  Files         240      240              
  Lines       34992    35024      +32     
==========================================
+ Hits        31582    31617      +35     
+ Misses       3410     3407       -3     

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