What changed

Added a focused interpreter regression test for nested function returns in /Users/siyulpark/.codex/worktrees/d9b2/minivm/interp/interp_test.go.

The new case exercises a recent interp/threaded.go change that replaced repeated frames[fp-1] lookups with the cached current-frame pointer i.fr. It verifies that after an inner function returns, the caller frame is restored before the caller executes LOCAL_SET and LOCAL_GET.

Changed paths tested

• interp/threaded.go: CALL/RETURN frame restoration through subsequent caller-local access
• interp/interp_test.go: added nested return restores caller frame for locals

Validation

• go test ./interp -run 'TestInterpreter_Run/nested return restores caller frame for locals'
• go test ./interp -run 'TestInterpreter_Run|TestInterpreter_WithThreshold'

Skipped targets

• Broad opcode re-coverage in interp/threaded.go: skipped because the recent refactor mostly changed frame-pointer access patterns, and existing table-driven runtime coverage already exercises many opcodes.
• JIT-only coverage: skipped because this change was isolated to threaded execution and the smallest reliable regression target is the threaded nested-return path.