feat(llm): LLM Provider Abstraction Layer (Issue #99 - $600 Bounty)

[sirius-016]

Summary

Implements a complete LLM Provider Abstraction Layer as described in Issue #99.

What's included

Core Modules (7 new files + 1 modified)

• Lux.LLM.Provider — New behaviour with metadata/0 and health_check/0 callbacks, model info helpers
• Lux.LLM.Registry — ETS-backed provider registry (GenServer for writes, ETS for reads)
• Lux.LLM.Router — Intelligent routing with 5 strategies: :cost, :latency, :quality, :round_robin, :fallback
• Lux.LLM.Fallback — Circuit breaker (closed/open/half_open states) + exponential backoff retry
• Lux.LLM.Monitoring — Per-provider & per-model cost/latency tracking with ETS aggregation
• Lux.LLM.Cache — ETS LRU cache with TTL, hit/miss statistics
• Lux.LLM.Manager — High-level chat/3 API that orchestrates routing + fallback + caching + monitoring
• Lux.LLM — Updated with chat/3, configure/1, usage_report/0, total_cost/0, cache_stats/0

Tests (7 files)

• Unit tests for Provider, Registry, Router, Fallback, Monitoring, Cache, Manager

Documentation

• Livebook guide (guides/llm_abstraction.livemd)
• Benchee benchmarks (benchmarks/llm_benchmark.exs)

Acceptance Criteria

• Universal provider interface (Lux.LLM.Provider behaviour)
• Provider registry and management system (Lux.LLM.Registry)
• Automatic model selection logic (Lux.LLM.Router with cost/latency/quality strategies)
• Intelligent fallback handling (circuit breaker + exponential backoff)
• Cost tracking and optimization (Lux.LLM.Monitoring with per-token pricing)
• Performance monitoring and analytics (latency, request counts, usage reports)
• Documentation and examples (Livebook guide, inline docs, benchmarks)

Total: 17 files, ~50KB

Closes #99