feat: M3.8 pluggable optimizer service (DRY + SOLID, 13 tests)

Refactored M3.8 to be extensible and customizable:

SOLID Architecture:
- Single Responsibility: OptimizerPlugin (optimize), FormatHandler (format)
- Open/Closed: Registry trait for extensibility without modification
- Liskov Substitution: Generic SimpleRegistry<T> works for any plugin type
- Interface Segregation: Traits focused, minimal methods
- Dependency Inversion: OptimizerService depends on abstractions

DRY Improvements:
- Generic Registry<T> trait eliminates duplicate register/get/list code
- PluginLocator strategy pattern replaces duplicated lookup logic
- OptimizerServiceBuilder factory pattern for ergonomic creation

Features:
✓ OptimizerPlugin trait (async optimization with metrics)
✓ FormatHandler trait (json, jsonl, raw, csv, yaml)
✓ Registry<T> generic trait (reusable for any plugin type)
✓ PluginLocator strategy (find optimizer by type, format by name)
✓ OptimizerService (orchestrator + dependency injection)
✓ OptimizerServiceBuilder (fluent builder)
✓ BuiltinOptimizer (wraps ContextOptimizer)
✓ 5 format handlers (JSON, JSONL, Raw, CSV, YAML)

Tests (13 passing):
- Registry registration and lookup
- Type-based optimizer finding
- Format handler discovery
- Service creation via builder
- Service optimization workflow
- Error handling on missing formats

Build: ✅ mem-core clean (130 tests total)

Usage:
  let service = OptimizerServiceBuilder::new()
      .with_optimizer(Arc::new(MyOptimizer))
      .with_format(Arc::new(JsonFormatter))
      .build()?;

  let output = service.optimize(content, "text/plain", Some("json")).await?;

Ready for:
- Custom optimizer implementations
- Custom format handlers
- Query optimization (next commit)
- Ingest pipeline integration (next commit)

crates/mem-core/src/optimizer/mod.rs

@@ -11,6 +11,8 @@ pub mod diff;
 pub mod text;
 pub mod cache_align;
 pub mod ccr;
+pub mod plugin;
+pub mod builtin;
 
 use anyhow::Result;
 use serde::{Deserialize, Serialize};
@@ -22,6 +24,13 @@ pub use diff::DiffCompressor;
 pub use text::TextCompressor;
 pub use cache_align::{CacheAligner, AlignedContent};
 pub use ccr::CcrStore;
+pub use plugin::{
+    OptimizerPlugin, FormatHandler, Registry, OptimizationResult, PluginMetrics,
+    SimpleRegistry, PluginLocator, DefaultLocator, OptimizerService, OptimizerServiceBuilder,
+};
+pub use builtin::{
+    BuiltinOptimizer, JsonFormatter, JsonlFormatter, RawFormatter, CsvFormatter, YamlFormatter,
+};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct OptimizedChunk {