feat: M3.8.1 phase 2 — JSON + Diff compressors (15 tests)

JsonCrusher (300 LOC):
- Field variance analysis for mid-array selection
- Allocation: 30% start (schema), 15% end (recency), 55% importance
- Truncates long strings (>500 chars) with markers
- Handles nested structures recursively

DiffCompressor (180 LOC):
- Keeps: file headers, hunk markers (@@), change lines (+/-)
- Drops: context lines (spaces), unchanged content
- Preserves binary file markers

32 optimizer tests total (17 phase1 + 15 phase2):
- JsonCrusher: 8 tests (object, array, boundaries, truncation, nesting)
- DiffCompressor: 7 tests (simple, multiple hunks, new/deleted files)

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

@@ -6,12 +6,16 @@
 
 pub mod router;
 pub mod log;
+pub mod json;
+pub mod diff;
 
 use anyhow::Result;
 use serde::{Deserialize, Serialize};
 
 pub use router::ContentRouter;
 pub use log::LogCompressor;
+pub use json::JsonCrusher;
+pub use diff::DiffCompressor;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct OptimizedChunk {
@@ -78,6 +82,8 @@ impl Default for ContextOptimizerConfig {
 pub struct ContextOptimizer {
     router: ContentRouter,
     log_compressor: LogCompressor,
+    json_crusher: JsonCrusher,
+    diff_compressor: DiffCompressor,
     config: ContextOptimizerConfig,
 }
 
@@ -91,10 +97,14 @@ impl ContextOptimizer {
     pub fn with_config(config: ContextOptimizerConfig) -> Result<Self> {
         let router = ContentRouter::new()?;
         let log_compressor = LogCompressor::new();
+        let json_crusher = JsonCrusher::new();
+        let diff_compressor = DiffCompressor::new();
 
         Ok(Self {
             router,
             log_compressor,
+            json_crusher,
+            diff_compressor,
             config,
         })
     }
@@ -121,7 +131,13 @@ impl ContextOptimizer {
             ContentType::Log if self.config.compress_logs => {
                 self.log_compressor.compress(content)?
             }
-            _ => content.to_string(), // TODO: Add other compressors
+            ContentType::Json if self.config.compress_json => {
+                self.json_crusher.compress(content)?
+            }
+            ContentType::Diff if self.config.compress_diff => {
+                self.diff_compressor.compress(content)?
+            }
+            _ => content.to_string(), // Passthrough for other types
         };
 
         let original_tokens = estimate_tokens(content);