[Architecture][P2] Phase 2: Decompose terraphim_service by capability

Parent: https://github.com/terraphim/terraphim-ai/issues/520
Related plan: docs/architecture-improvement-plan.md

Goal

Decompose terraphim_service into capability-focused modules with stable trait boundaries so logic is reusable and easier to test.

Deliverables

• Capability modules for:
  • search service
  • KG/thesaurus service
  • document service
  • LLM/chat service
• Stable crate facade with explicit interfaces and reduced monolithic lib.rs

Tasks

• Split branching workflows into strategy objects or focused modules
• Introduce typed service interfaces for integration and adapter reuse
• Keep external API compatibility while refactoring internals
• Add isolated tests per capability module

Acceptance Criteria

• crates/terraphim_service/src/lib.rs is primarily a facade/wiring layer
• Each service capability has isolated tests and explicit dependency boundaries
• Existing public consumers compile with no behavioral regression

Read docs/architecture-improvement-plan.md and inspect crates/terraphim_service/src/lib.rs to map the existing workflows and public consumers. Define capability boundaries and isolated tests for each listed service while preserving external API compatibility; done means lib.rs is mainly facade wiring, boundaries are explicit, and existing consumers compile without behavioral regression.