generic types, traits and lifetimes

- get claude to create rustlings style exercises based on chapter 10 of the book
- update CLAUDE.md to prevent it from producing incorrect exercises
- complete exercises/01-generic-cache-system

CLAUDE.md

@@ -35,6 +35,31 @@
 **Stretch goal:** [Optional extension]
 ```
 
+### Exercise Validation (CRITICAL)
+**BEFORE providing any exercise, I MUST:**
+1. **Mental trace-through** - Mentally implement the solution to verify it's solvable
+2. **Signature verification** - Check all function signatures are correct for the requirements
+3. **Trait bound validation** - Ensure trait bounds match what the implementation actually needs
+4. **Test compatibility** - Verify the test cases will work with correct implementations
+5. **Compilation check** - Ensure starter code compiles (even with todo!() placeholders)
+
+### File Structure (REQUIRED - Rustlings Style)
+**README.md contains:**
+- 2-3 sentence concept explanation (like Rustlings)
+- What you'll practice (bullet points)
+- Links to relevant documentation
+- KEEP IT MINIMAL - no dense walls of text
+- NO starter code templates or expected output
+
+**Code files contain:**
+- ONLY clean starter code with correct function signatures
+- ONLY necessary imports and trait bounds
+- ONLY test functions using #[cfg(test)] for verification
+- Minimal TODO comments explaining what to implement
+- NO explanations or tutorials in comments
+
+**Rustlings principle:** Minimal README, focus on code. Student learns by making tests pass.
+
 ### Exercise Types Per Milestone
 - 2 general exercises (core programming concepts)
 - 1 crypto-specific exercise (domain building)
@@ -76,6 +101,8 @@
 
 ## Learning Constraints to Respect
 - Student quits when frustrated - keep exercises achievable
+- **Incorrect exercise design destroys confidence - ALWAYS validate before providing**
+- **Unsolvable exercises are worse than no exercises - verify signatures and bounds**
 - Avoid "copy-paste progress" - ensure understanding
 - Exercises must be tightly coupled to building (not random drills)
 - Prefer modifying existing code over starting from scratch