Deprecate df_state interface

[sunt05]

Parent Epic

Part of #1045 (Epic: DTS Accessor Interface - Eliminate df_state Intermediate Layer)

Summary

Final phase: deprecate and eventually remove the df_state intermediate layer once DTS path is fully mature.

Depends On

• DTS core infrastructure with single-grid runner and SUEWSSimulation integration #1072 (DTS core infrastructure)
• Multi-grid support
• Chunked simulation support
• Documentation complete
• Sufficient user migration period

Scope

Phase 1: Deprecation Warnings

• Add deprecation warnings to to_df_state() methods
• Add deprecation warnings to df_state usage in run()
• Document migration path

Phase 2: Default Switch

• Change default backend from "traditional" to "dts"
• Maintain backwards compatibility via explicit backend="traditional"

Phase 3: Removal

• Remove to_df_state() methods (49+ methods across 8 files)
• Remove df_state related code
• Clean up DataFrame conversion utilities

Acceptance Criteria

• Deprecation warnings in place with clear migration message
• Users have at least one release cycle to migrate
• All functionality available via DTS path before removal
• Clean removal of ~2000+ lines of redundant code