refactor: improve diagnostic message for unreplaceable working variables

refactor(MmpValidator): extract diagnostic logic for complete proof with working vars that cannot be automatically replaced

refactor(mmp): extract proof statement generation logic

Handle missing unused variables during working var replacement

test: remove unused variables from impbii.mm

refactor: rename WorkingVarReplacer to WorkingVarReplacerForCompleteProof

Rename WorkingVarReplacer to clarify its specific purpose

test: add regression test for multiple working variables (Issue #23)

Refactor proof statement generation to separate concerns

feat: replace working variables with theory variables on proof completion

test: add failing case for working variable replacement (#23)

feat: support MMP conversion for normal (non-compressed) Metamath proofs (Correction to previous commit message)

Fix comment handling and compressed proof support in MmToMmpConverter

Merge pull request #28 from Antony74/feature/mm-parser-flexibility

protected buildLabelToStatementMap for testing