Handlebars: optimize render state

[alandefreitas]

RenderState keeps a vector of every ancestor level. Helpers only need to walk up the stack when .. appears, so the vector incurs unnecessary allocations.

Proposed solution

• Store only the current level plus a pointer to the parent state.
• Follow parent pointers when .. is used repeatedly.
• Remove vector allocations and simplify debug output.

Requirements

• Engine keeps only the current state and a parent pointer.
• Helpers behave the same.
• Benchmarks show reduced allocations.
• Documentation/comments explain the new layout.

Note
Once the standalone Handlebars repository exists, move this issue there and track it in the new project.