Expose shared Servo thread logs to skit host logging

[staging-devin-ai-integration]

Summary

The native Servo plugin's shared Servo thread emits diagnostics with tracing, but the plugin dylib has no tracing subscriber. As a result, all shared-thread diagnostics are silently dropped before reaching skit, including the CSS-stage log introduced while addressing #661.

This is especially harmful for production diagnosis because the dropped error-level messages include instance poisoning and Servo/render panic failures.

Current scope

plugins/native/servo/src/servo_thread.rs has 18 tracing calls:

• 8 tracing::error!
• 7 tracing::info!
• 2 tracing::debug!
• 1 tracing::warn!

All 18 are currently invisible in skit logs.

Proposed fix

Thread the host logger through shared-thread registration:

1. Clone the SDK Logger into ServoWorkItem::Register.
2. Store the logger in each shared-thread InstanceState.
3. Replace the shared-thread diagnostics with the SDK plugin_* macros so messages use the host callback and appear in skit.

The error-level poison/panic diagnostics are the highest-priority messages to preserve.

Context

The CSS-stage logging issue was discovered while working on web-capture PR #661. The line was intentionally left consistent with the existing file convention rather than partially converting only one call site.

[staging-devin-ai-integration]

Fixed by #668 — shared Servo thread diagnostics now route through the host Logger and appear in the skit log with [node_id] prefixes.