peer: route the peer-box sing-box logger into radiance's slog

[myleshorton]

Summary

The second sing-box instance the peer client spins up was constructed with a plain `box.BaseContext()` and no sing-box log factory registered. That means router / dial / outbound errors from the peer-box landed on the box's default stderr logger and never reached `lantern.log` — which made the SmC verify-failure incident triaged today unnecessarily hard.

We had:

• `[samizdat] CONNECT api.iantem.io:443: handler started` at `20:41:35.079`
• `[samizdat] CONNECT api.iantem.io:443: handler returned, starting drain` at `20:41:35.096` — 17 ms later
• Then 5 s of drain timeout and 1475 bytes of the verifier's TLS ClientHello discarded
• Server-side: `peer verify failed: verify round-trip through peer: Get "https://api.iantem.io/v1/peer/verify-callback?token=...\": EOF`

The peer's sing-box outbound dial to `api.iantem.io` failed almost instantly — but with zero error log anywhere in either `lantern.log` or `lantern_macos.log`. The `i.logger.ErrorContext(ctx, err)` at `lantern-box/protocol/samizdat/inbound.go:178` would have caught the underlying error, but the peer-box's logger output goes to a default stderr destination instead of slog.

Change

Mirror what `vpn/tunnel.go:141-142` already does for the main tunnel's box — register `lblog.NewFactory(slog.Default().Handler())` as the `sblog.Factory` on the peer-box's context before `NewServiceWithContext`. sing-box logger output now flows through radiance's slog and into `lantern.log` alongside everything else.

Test plan

• `go build ./peer/...` passes
• `go test -count=1 ./peer/...` passes (existing peer tests still green)
• Next SmC toggle that triggers a verify failure should now produce a sing-box-side error log line in `lantern.log` explaining the actual dial failure — that's the signal we were missing today.

Stack note

Branched off `fisk/peer-connection-events` because the peer module lives there. Will rebase onto main once the peer module merges.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR routes the peer client’s secondary sing-box instance logging through Radiance’s slog pipeline so peer-box router/dial/outbound errors reach lantern.log.

Changes:

• Adds sing-box service/log imports and lantern-box log factory import.
• Registers an sblog.Factory on the peer box context before creating the libbox service.

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