Allow callers to plug a custom dialer into the smart strategy

[myleshorton]

The Outline-SDK smart strategy in `WithProxyless` / `NewSmartHTTPTransport` was hardcoding `transport.TCPDialer{}` / `transport.UDPDialer{}` as its base dialers. Both wrap the stdlib `net.Dialer`, so on a host with an active VPN TUN claiming the default route the smart-strategy probes (and the eventual production dial) get captured by the TUN and loop through the caller's own tunnel.

Radiance currently works around this by overwriting the returned `http.Transport`'s `DialContext` with a bypass dial — but that throws away the entire smart strategy. The smart-dialer cost is paid at construction and the strategy is then never actually invoked at request time, so kindling traffic gets neither DPI evasion nor the VPN-loop fix together.

API additions

Two new options on the Kindling instance:

```go
WithStreamDialer(d transport.StreamDialer) Option
WithPacketDialer(d transport.PacketDialer) Option
```

And a new standalone helper that mirrors `NewSmartHTTPTransport`:

```go
NewSmartHTTPTransportWithDialer(
logWriter io.Writer,
stream transport.StreamDialer,
packet transport.PacketDialer,
domains ...string,
) (*http.Transport, error)
```

When the supplied dialer is `nil`, `newSmartDialer` falls back to the original `TCPDialer{}` / `UDPDialer{}`, so existing callers (anything using `WithProxyless(domains...)` or `NewSmartHTTPTransport(logWriter, domains...)` without setting the new options) get exactly today's behavior.

Why now

Surfaced in a Slack thread from Patrick on 2026-05-15: radiance can't actually feed its bypass dialer into the smart strategy. The workaround it uses (override DialContext post-hoc) suppresses the strategy entirely.

After this PR, radiance can wire `bypass.DialContext` into a `transport.StreamDialer` adapter and pass it via `NewSmartHTTPTransportWithDialer`, restoring the smart strategy on top of bypass.

Other strategies

strategy	bypass plumbing	status before this PR
domainfront	`domainfront.WithDialer(...)`	already correct
amp	`amp.WithDialer(...)`	already correct
smart / proxyless	hardcoded `transport.TCPDialer{}`	fixed here
dnstt	private `dohDialer` / `dotDialer` types	same problem; needs a separate fix in dnstt itself, out of scope here

Test plan

• `go build ./...`
• `go test ./...` (existing tests pass)
• Follow-up radiance PR rewires `kindling/smart/client.go` to use `NewSmartHTTPTransportWithDialer` with a `bypass.DialContext`-backed `StreamDialer` adapter
• Verify on a real device with VPN active: smart-strategy probes appear in logs (currently they don't because radiance overrides DialContext) and connection counters on the bypass listener increment

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR adds support for supplying custom base dialers to the Outline SDK smart/proxyless strategy so callers can bypass host routing, such as VPN TUN routing, while still using smart dialing.

Changes:

• Adds WithStreamDialer and WithPacketDialer options for Kindling smart/proxyless transports.
• Updates smart dialer construction to use custom stream/packet dialers with default fallbacks.
• Adds NewSmartHTTPTransportWithDialer as a standalone helper mirroring NewSmartHTTPTransport.

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