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[Grpc] fix(#81): Bound the outbound drain by transport writability (#82)

# Description

`GrpcBridge` applied flow control to the inbound side of a call only.
The buffered path requests one
message at a time and the streaming path requests up to
`maxInboundMessages` and refills one per
drained message, so a client cannot flood the server. Outbound had no
counterpart — no `isReady()`,
no `onReady`, no readiness check anywhere in the bridge — so a
server-streaming handler wrote as fast
as it yielded, and grpc-java queued everything a stalled peer had not
read. A large response to a
client that stops reading grew the transport's write queue for the life
of the call. This bounds the
other direction.

Closes #81

## The contract question: this is adapter-local, not agnostic

The issue asks whether outbound readiness belongs on the worker-agnostic
surface. It does not, and
nothing in `GRPC.md` / `GRPC_IMPLEMENTATION.md` or any sibling port
needs to change.

The agnostic surface already hands the adapter both places it needs to
pause:

- **Buffered model** — outbound messages are a *pull* iterable drained
through `call.cancellable(...)`.
The adapter controls when it pulls the next item, so "wait for
writability" is just "don't pull yet."
- **Streaming model** — `OutboundStreamContract.sendMessage` is a
synchronous call the adapter
  implements. "Wait for writability" is just "don't return yet."

`GRPC.md` step 7 already assigns the adapter the job of translating a
`ServiceResponse` into the
library's native response API; pacing that translation to the transport
is the same job. Hoisting
readiness onto the contract would mean inventing a portable spelling for
a concept whose primitives
genuinely differ per library (`isReady`/`onReady` in grpc-java,
`writable`/`drain` on a Node stream,
channel-send blocking in Go) in order to express a behavior each port
can already produce without it.
The portable *semantics* — the drain pauses between messages — is
reachable... (continued)

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