Add metrics and graph transports

[nikhilvdev]

Add a MetricsPlugin with OpenTelemetry, StatsD, and Prometheus Pushgateway backends, plus batched Neo4j, FalkorDB, and Neptune transports. Reorganize core, runtime, tracing, CLI, and privacy modules into themed subpackages and update imports, docs, packaging, and tests.

Summary

Checklist

• ruff check . passes
• mypy logquill passes (--strict on the public API)
• pytest passes, and new surface area has test coverage
• Public API additions are documented in README.md with a runnable example
• CHANGELOG.md has an entry under Unreleased
• If this touches the cross-language contract (levels, logger methods,
  record shape, transport/plugin contracts, config shape) shared with
  logquill-js, a matching tracking issue/PR is opened there

Scope