fix: ThreadSafeDeque race condition in kill_all()/notify_all() (#298)

* fix: ThreadSafeDeque race condition in kill_all()/notify_all()

Fix a race condition in ThreadSafeDeque where kill_all() and notify_all()
modified _is_waitable and signaled the condition variable without holding
the mutex. This caused a deadlock when the consumer thread (read_row)
checked is_waitable()==true but the worker thread called kill_all() before
the consumer entered wait(), causing the notify signal to be lost.

Timeline of the race:
  1. Consumer: records->empty() == true
  2. Consumer: records->is_waitable() == true  (lock-free atomic read)
  3. Worker:   kill_all() sets _is_waitable=false, calls _cond.notify_all()
  4. Consumer: wait() acquires lock, enters _cond.wait() - DEADLOCK
     (notification was already sent in step 3, predicate !is_waitable()
      was already true but checked AFTER the signal was lost)

Fix: Acquire the mutex in kill_all() and notify_all() before modifying
_is_waitable and signaling. This ensures the condition variable
notification cannot be lost between the is_waitable() check and wait().

The bug is most easily triggered with small CSV files where the worker
thread completes almost instantly, and is particularly reproducible in
no_header() mode on Windows.

Also adds a regression test (test_threadsafe_deque_race.cpp) that
exercises the race window with rapid small-CSV iterations.

* fix: namespace pollution in string_view header

Move C++ standard detection and string_view includes before csv namespace
to prevent <functional> from being included inside csv namespace.

This prevents namespace collision where std:: gets resolved as csv::std::

* Added docs for TSD, timeout to tests

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Co-authored-by: Vincent La <vincela9@gmail.com>

Author: YamingPei