Give each SSL call its own byte count

[krynju]

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

Bug. readbytes, writebytes, peekbuf and peekbytes are Refs on the stream, shared by every task using it, and read back after @geterror has released ssl.lock. Another task's SSL call can overwrite the count in that gap. A writer that reads back a smaller count than its own thinks its chunk was not written and submits most of it again, duplicating plaintext on the wire; reads and peeks have the same hole. On main the gap is the lock release just before the read-back, so it needs threads to hit; any change that moves the socket write out from under the lock (the #72 fix) widens it to every parked write.

Fix. Each call allocates its own Refs. The four fields stay on the struct, unused by this package, since code outside it reads transport.readbytes[].

Test. ConcurrentWriters: four writers with different chunk sizes on one stream, the server checks every writer's bytes arrived exactly once. It passes on main on one thread, where the window cannot be hit, and guards the invariant from here on.

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 78.96%. Comparing base (10f7b7b) to head (03d7af9).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #88      +/-   ##
==========================================
+ Coverage   77.56%   78.96%   +1.39%     
==========================================
  Files           2        2              
  Lines        1083     1193     +110     
==========================================
+ Hits          840      942     +102     
- Misses        243      251       +8     

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