Codebase Modularization (Phased)

This release includes internal refactors to modularize several monolithic files into focused packages while preserving the public API.

Highlights: - Scheduler split into meridian.core.scheduler package (config, coordination, execution, fairness, shutdown) - Observability split across packages: meridian.observability.{tracing,metrics,logging} - Core graph components split: meridian.core.{node,subgraph,edge} packages - Utilities: meridian.core.{priority_queue,runtime_plan,policies} packages

Backwards compatibility: - All public imports from meridian.core and meridian.observability continue to work via re-exports - Added import compatibility tests in the test suite

Migration notes: - No breaking changes expected. If deep internal imports were used, prefer the package entry points.

Owner: GhostWeasel