Add WithDialer option for custom TCP dialer injection

[myleshorton]

Summary

• Introduces a DialFunc type and WithDialer functional option that allows callers to inject a custom TCP dialer into the fronted package, overriding the default net.Dialer
• Threads the custom dialer through from the top-level fronted struct down to individual front instances so it is used for all underlying TCP connections
• This is needed for routing fronted traffic through a bypass proxy to avoid VPN tunnel loops (see feat: local bypass proxy for kindling traffic radiance#341)

Test plan

• TestWithDialer verifies that a custom dialer is stored and invoked when set
• TestWithDialerDefault verifies a default dialer is assigned when WithDialer is not used
• TestWithDialerFlowsToFronts verifies the custom dialer propagates from the top-level config down to individual fronts and is used during dial()
• Manual integration testing with the radiance bypass proxy to confirm fronted traffic routes outside the VPN tunnel

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR introduces the ability to inject a custom TCP dialer into the fronted package, enabling users to control how the underlying network connections are established. This is particularly useful for routing fronted traffic through a bypass proxy to avoid VPN tunnel loops.

Changes:

• Adds a DialFunc type and WithDialer functional option for custom dialer injection
• Threads the custom dialer from the top-level fronted struct through to individual front instances via function parameter passing
• Includes comprehensive tests verifying custom dialer storage, default dialer fallback, and proper propagation through the call chain

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
fronted.go	Defines DialFunc type, adds dialFunc field to fronted struct, implements WithDialer option, sets default dialer in NewFronted, and threads dialer to loadFronts
front.go	Adds dialFunc field to front struct, updates newFront signature to accept dialer, and modifies dial() method to use custom dialer with nil fallback
fronted_test.go	Adds three comprehensive tests for custom dialer functionality and updates existing TestLoadFronts to pass nil dialer parameter

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