StringRef ref counting and proper deallocation

[dmgcodevil]

Implemented a thread-safe intrusive reference counting system for StringRef to solve critical concurrent string access issues in TundraDB. This replaces the previous unsafe string management approach that risked use-after-free bugs during concurrent node updates.

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Problem Statement

Before this PR:

• ❌ No automatic string lifetime management
• ❌ Risk of use-after-free during concurrent updates
• ❌ Manual deallocation required (error-prone)
• ❌ No protection against dangling pointers
• ❌ Reference counting required expensive map lookups

Specific issue:

// Thread 1: Reading node field
StringRef name = node->get_field("name");

// Thread 2: Updates same field
node->set_field("name", new_value);
// ❌ Old string immediately deallocated
// ❌ Thread 1's StringRef now dangling! 💥

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Solution

Implemented intrusive reference counting where the ref count is stored IN the arena memory, directly before the string data. This provides:

1. Automatic Memory Management: Strings deallocated when last reference destroyed
2. Two-Phase Deletion: Mark for deletion → deallocate when safe
3. Zero-Lookup Ref Counting: No map overhead, just pointer arithmetic
4. Thread-Safe: Lock-free atomic operations for ref counting
5. Cache-Friendly: Header adjacent to string data (single cache line)

Memory Layout:

Arena Memory:
┌────────────┬────────┬─────────┬─────────┬──────────────────┐
│ ref_count  │ length │  flags  │ padding │  string data...  │
│   (4B)     │  (4B)  │  (4B)   │  (4B)   │                  │
└────────────┴────────┴─────────┴─────────┴──────────────────┘
      ↑                                    ↑
   Header (16 bytes)                   StringRef.data_ points here

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Architecture Changes

New Clean Header Structure:

value_type.hpp        ← ValueType enum + type system helpers (no dependencies)
    ↓
string_ref.hpp        ← StringRef class (intrusive ref counting)
    ↓
string_arena.hpp      ← StringPool + StringArena (string storage)
    ↓
types.hpp             ← Value + ValueRef (value containers)