drpcstream: add per-stream send-window credit primitive

[suj-krishnan]

Adds sendWindow, the per-stream flow-control credit balance on the sender:

• acquire(ctx, n) — spends credit, blocking until enough is available; interruptible by both context cancellation and close, consuming no credit when it bails.
• grant(n) — strictly additive (no-revoke); wakes a parked acquirer.
• close(err) — terminates the window, waking all waiters with the error.

A grant only ever adds credit; the receiver never takes back credit it has already issued. The balance is a signed int64: the flow control enablement design (pre-armed accounting, see the design doc) has the sender debit bytes it sends before flow control begins enforcing, which can leave the balance negative — a stream then repays that deficit from incoming grants before its next message. acquire blocks whenever available credit is below what the next frame needs, negative balances included.

Scope: the primitive only. It is not yet wired into the stream write path.

Tests: immediate acquire, grants accumulate, blocks-until-grant, context-cancel wake, close wake, acquire-after-close. All pass under -race.

[shubhamdhama]

Rest LGTM.

[shubhamdhama]

redundant

[shubhamdhama]

Employ something like

drpc/drpcwire/mux_writer.go

Lines 76 to 85 in cbabd21

	// unblockWritesLocked wakes every producer parked on backpressure by closing
	// the current drain channel (a broadcast) and installing a fresh one. The
	// blocked guard keeps this allocation-free on the common path where nobody is
	// waiting. It must be called with mu held.
	func (mw *MuxWriter) unblockWritesLocked() {
	if mw.blocked > 0 {
	close(mw.drain)
	mw.drain = make(chan struct{})
	}
	}

[suj-krishnan]

You mean, add the check so we don't allocate when there are no waiters? I didn't keep it because grants are not as frequent (we coalesce and grant only for every 1 MB or so) - keeping a blocked counter and updating in every case didn't seem worth it. (The mux_writer logic is clearly in the hot path and needs it).
I will use a nil check on the channel instead to save on the allocation.

[suj-krishnan]

TFTR @shubhamdhama!
Merging this PR - the code in it is never exercised and therefore safe across drpc version bumps.