Wire the sendWindow credit gate into rawWriteLocked: a KindMessage frame
acquires len(frame) bytes of per-stream send credit before it is handed
to the writer. Control frames (invoke/metadata) bypass the gate.

The window is opt-in: a stream has no send window by default, so data
writes stay ungated (unlimited) and behavior is unchanged until one is
installed. terminate closes the window with the send-side error
(sigs.send is first-wins, holding io.EOF when a cancel/error path
pre-set it), so a send parked on credit returns the same error as one
parked in WriteFrame or a later send. SendCancel and Cancel already
terminate before taking the write lock, so they wake a credit-parked
writer.

acquire no longer takes a context: the stream's context Done channel is
only closed once the stream is finished (after all ops complete), which
cannot happen while an acquire is parked (the parked write is itself an
in-flight op), so it never fired there. close is the sole abort path,
and every termination route reaches it.

Per-stream only. NOTE: Close/SendError/CloseSend still take the write
lock while holding s.mu, so a send parked on credit can deadlock them;
the lock-ordering rework that makes those paths preempt a parked writer
is handled separately.

Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>