feat(client): add transport health telemetry

[aej]

Problem

Today an observability library can see RPC and logical connection events, but it cannot reliably answer basic transport questions: is the HTTP/2 connection up, did it reconnect, how many streams are active, or has the peer's stream limit been reached?

The only way to get those facts is to inspect Gun/Mint processes or other private state. I don't think observability code should depend on those implementation details.

What this adds

This adds a transport-neutral [:grpc, :client, :transport, ...] telemetry API for both Gun and Mint:

Event	Meaning
:started	A physical transport has been created but is not usable yet. Generation and active streams both start at zero.
:connected	The transport is usable. Includes the generation and whether this was a reconnect.
:connect_error	Connection establishment failed at a reliably known resolve, tcp, tls, or http2 stage.
:down	An established transport became unavailable. This event owns the count of streams terminated by the loss.
:stopped	The transport was permanently removed or explicitly shut down. This is separate from a temporary :down.
:settings	The peer's effective SETTINGS_MAX_CONCURRENT_STREAMS changed, or the value is not safely known.
:streams	An authoritative active-stream snapshot after the transport's stream table changes.

It also adds optional failure_stage and failure_reason fields to existing client RPC telemetry where the cause is known, for example a deadline, unavailable connection, or stream error.

The intention is that a consumer can calculate logical readiness and capacity across multiple physical transports without using connection references as metric labels.

Deliberate limits

Mint also reports 100 both before an explicit peer limit is known and when the peer really advertises 100. This PR reports that case as :unknown rather than inventing finite capacity. A small upstream Mint hook exposing whether the setting was present would close that gap.

Connect failures are only given a stage (resolve, tcp, tls, or http2) when the adapter exposes that reliably. Error text is not parsed.

Testing

The new events use temporary telemetry handlers in tests