drpcstream: byte-account the per-stream receive queue

[suj-krishnan]

Stacked on #92 (base: sujatha/flow-control-overdraft).

Problem

The receive queue (ringBuffer) bounded the producer by message-slot count — a fixed 256-slot ring blocking on count == len(buf). Slot-based bounding makes the flow-control memory bound a fiction: a slow consumer can buffer 256 arbitrarily large messages, so the "receiver holds ≤ StreamWindow + MaxMessageSize" guarantee doesn't hold. Grants are already byte-accurate (recvw.consumed(len)); the queue that holds those bytes was not.

Change

Bound and account the queue in bytes when flow control is on:

• Budget = StreamWindow + MaxMessageSize — the per-stream receive peak (a window of un-granted bytes plus one message overdrafting to finish).
• The budget is denominated in payload bytes — the same unit the sender debits and grants return — so a conforming sender within its wire credit can never fill the queue and block the shared connection reader (manageReader).
• Admission (admits) folds in the incoming message's length with an empty-queue exception, so the queue never overshoots by a message and an oversized message is still deliverable.
• The pooled copy is made after admission, so a blocked Enqueue holds no buffer outside the accounted queue.
• Under a budget the ring grows past its initial capacity, so message count never blocks the producer. Flow control off keeps the legacy 256-slot bound unchanged.

Deferred

• Ownership transfer (drop the Enqueue copy and the assembler's retained backing array) — closes the accounted-vs-retained gap; a follow-up.
• Zero/tiny-credit (empty) message floods — a payload-aligned queue cannot cap these without breaking liveness, because an empty message consumes no wire credit. The liveness-safe fix is a per-message minimum charged symmetrically in send credit, grants, and the queue.

Testing

go test ./drpcstream/ ./drpcmanager/ ./drpcwire/ -race (and the full suite) pass; gofmt clean. New ring-buffer tests cover byte-budget blocking/unblocking, incoming-length admission, byte release on dispatch, oversized-on-empty
acceptance, and ring growth; a stream-level test asserts the budget is StreamWindow + MaxMessageSize (and unset when flow control is off).

Part of CRDB-65748.