feat: implement LEAVE and UNLOOP words

[tetsuo-cpp]

Summary

• Implement LEAVE and UNLOOP in the Forth-to-MLIR translator.
• Fix conditional LEAVE semantics in DO/LOOP so the backedge is preserved for non-LEAVE paths.
• Add translation, conversion, and pipeline tests for LEAVE plus UNLOOP error handling.
• Add a translation regression test for conditional LEAVE.

Tests

• cmake --build build --target check-warpforth

Implementation Notes

• LEAVE lowers to a cf.cond_br with a constant true to the loop exit and a dead block that carries the stack argument, avoiding type-conversion mismatches.
• LOOP always emits the counter increment and backedge; only the block executing LEAVE branches to the loop exit.
• UNLOOP pops the innermost loop context and falls through; it errors if used without a matching DO.

Closes #23.

[tetsuo-cpp]

The current leaveDepth gate in LOOP disables the counter increment/backedge for the entire loop whenever any LEAVE appears, which changes semantics for conditional LEAVE (loop exits after first iteration even when condition is false). The backedge should still be emitted for non-LEAVE paths; only the block that executes LEAVE should branch to loop-exit.