fix typo

perf(stream): cache window update threshold to reduce computation

Add windowUpdateThreshold field to stream struct to cache the
MaxStreamBuffer/2 calculation. This avoids repeated pointer
dereferences (s.sess.config.MaxStreamBuffer) and integer division
on every tryReadV2() and writeToV2() call in the hot path.

- Add windowUpdateThreshold uint32 field to stream struct
- Initialize threshold in newStream()
- Use cached value in tryReadV2() and writeToV2()

Merge branch 'dev'

Merge branch 'dev'

Merge branch 'dev'

Merge branch 'dev'

Squashed commit of the following:

commit c8a401f
Author: xtaci <imap@live.com>
Date:   Sat Jan 24 22:29:03 2026 +0800

    fix(session): reject cmdUPD in protocol version 1

    cmdUPD is a v2-only command used for flow control window updates.
    Previously, recvLoop would accept and process cmdUPD even when
    the session was configured for protocol version 1, which violates
    the protocol specification.

    This change adds a version check before processing cmdUPD. If the
    session is not running v2, it now correctly returns ErrInvalidProtocol
    and terminates the receive loop.

update header diagram

Fix memory leak caused by buffer capacity modification during read op…

…erations preventing successful allocator recycling.

Merge branch 'dev'