eof spun forever on a record truncated by the peer closing

[krynju]

Split out of #73, as asked in the review there: an independent fix, against main.

Bug. When the peer closes mid-record, SSL_peek_ex keeps returning WANT_READ, haspending stays true for the partial record, and eof(ssl.io) returns true at once. eof then loops without ever yielding, which starves every other task on the thread; on one thread the process hangs.

Fix. In the WANT_READ branch of eof, return true when the socket is at EOF: the bytes that would complete the record are never coming, so this is the end of the stream. One more round is allowed first (review finding): a concurrent unsafe_read waiting on the socket ahead of this task can take the rest of the record while eof waits, in which case bytesavailable shows the decrypted bytes on the next round and eof is false.

Test. TruncatedRecordEOF sends a record header announcing 100 bytes followed by 3 of them, then closes the socket with a FIN (a reader task keeps the client socket drained of the server's session tickets, so the close is not a RST). On main the test spins until the job timeout instead of failing.

The test file gains the same three helpers as #86 (self-signed server context, handshake loop, connected pair), in the same place, so whichever lands later only has to re-append its testset.

No version bump; the fixes can be released together.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.79%. Comparing base (10f7b7b) to head (0e75202).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
+ Coverage   77.56%   78.79%   +1.23%     
==========================================
  Files           2        2              
  Lines        1083     1193     +110     
==========================================
+ Hits          840      940     +100     
- Misses        243      253      +10     

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