Fix critical security and concurrency issues

[kriscoleman]

This commit addresses all critical issues identified in the code review:

🔒 SECURITY FIXES:

• Replace insecure math/rand with crypto/rand for jitter generation
• Ensures cryptographically secure randomness to prevent predictable patterns
• Eliminates race conditions in concurrent jitter usage

⚡ CONCURRENCY FIXES:

• Fix StopPolicy race condition by moving timing to retrier
• Remove shared state (startTime) from policy structs
• Ensure thread-safe operation across goroutines

🛡️ OVERFLOW PROTECTION:

• Add bounds checking in exponential backoff calculations
• Prevent integer overflow with large attempt numbers
• Graceful handling of infinite/NaN multiplier values

✅ INPUT VALIDATION:

• Add comprehensive parameter validation to all constructors
• Prevent negative delays, attempts, and invalid multipliers
• Clear panic messages for invalid inputs

🧪 ENHANCED TESTING:

• Add comprehensive validation tests (validation_test.go)
• Test overflow protection and security improvements
• Verify proper error handling and edge cases
• Update StopPolicy tests to reflect architectural changes

🔧 INFRASTRUCTURE:

• Fix timer leaks with proper cleanup on context cancellation
• Use time.NewTimer() with explicit Stop() calls
• Better error messages and code documentation

All tests pass (30/30) with no regressions.
Addresses GitHub issue #1.