feat(translate): implement EXIT word and fix UNLOOP for early loop exit

[tetsuo-cpp]

Summary

• Implement EXIT word for early return from user-defined words (colon definitions), emitting func.return via a dummy cond_br pattern to keep dead blocks structurally reachable for dialect conversion
• Fix UNLOOP to not pop the loop stack at parse time, enabling the idiomatic UNLOOP EXIT pattern for early loop exit
• Error on EXIT outside a word definition

Test plan

• Translation test: EXIT inside IF/THEN emits return (test/Translation/Forth/exit.forth)
• Negative test: EXIT outside word definition (test/Translation/Forth/exit-outside-word-error.forth)
• Translation test: UNLOOP EXIT inside DO LOOP (test/Translation/Forth/unloop-exit.forth)
• Pipeline test: EXIT compiles to gpu.binary (test/Pipeline/exit.forth)
• Pipeline test: UNLOOP EXIT compiles to gpu.binary (test/Pipeline/unloop-exit.forth)
• All 54 tests pass

Closes #24

[tetsuo-cpp]

Looks good! Clean implementation that follows the established LEAVE pattern well.

A few observations:

• UNLOOP fix is correct — pop_back_val() was desyncing the loopStack, this no-op approach is right since loop control is CFG blocks + memref counter.
• EXIT implementation mirrors LEAVE's cond_br %true → return/dead block pattern nicely. Good use of InsertionGuard.
• Test coverage is solid — translation, negative, pipeline, and the realistic FIND-FIVE idiom.

One thing to consider as a follow-up: EXIT inside a loop without UNLOOP (e.g. DO ... EXIT ... LOOP) silently leaks the loop counter alloca. Matches standard Forth semantics where UNLOOP is the programmer's responsibility, but a diagnostic warning could be helpful down the line.