Add with_tracing_filter for custom/reloadable tracing filters

[cetra3]

Summary

• Adds LogfireConfigBuilder::with_tracing_filter() method that accepts any F: Filter<Registry>, allowing consumers to replace the default EnvFilter with a custom filter (e.g. reload::Layer<EnvFilter> for dynamic reloading)
• Changes LogfireTracingLayer::new to accept Arc<dyn Filter<S>> instead of Arc<EnvFilter> (internal API only)
• No local() mode required — the custom filter integrates into the normal finish() flow, so the global subscriber, log bridge, console output, and meter provider all work as before

Test plan

• test_with_tracing_filter_custom_filter — verifies a static LevelFilter::WARN replaces the default, filtering out lower-level events
• test_with_tracing_filter_reload — verifies a reload::Layer<EnvFilter> can dynamically switch INFO→DEBUG→INFO, with events correctly filtered at each phase
• All existing tests pass

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.85185% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/bridges/tracing.rs	90.35%	11 Missing ⚠️

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[cetra3]

Closing this one off for now, it looks like this probably won't work for tracing & needs a bit of a refactor