feat(translate): implement +LOOP word for strided DO loops

[tetsuo-cpp]

Summary

• Extract common loop-end logic into emitLoopEnd helper
• Refactor LOOP to use the new helper
• Add +LOOP which pops an arbitrary step from the data stack

Test plan

• Translation test: plus-loop.forth
• Error test: plus-loop-without-do-error.forth
• Pipeline test: plus-loop.forth
• All 57 tests pass

Closes #25

[tetsuo-cpp]

Code Review

Clean, well-structured PR. The emitLoopEnd extraction is a natural refactoring that eliminates duplication without over-abstracting. The +LOOP implementation is minimal and easy to follow.

Tests

Thorough coverage — translation IR check, error case, and full pipeline test.

Two items to consider

1. Negative step values: Standard Forth +LOOP uses a direction-sensitive termination check — positive step terminates when index >= limit, negative step terminates when index < limit. The current implementation always uses slt, so negative step values will produce an infinite loop. Worth documenting the limitation or addressing it.

2. CLAUDE.md: The supported words list says DO LOOP I J K but doesn't mention +LOOP.