Reverse / bidirectional mapping

[sanelli]

Summary

Support reverse / bidirectional mapping so a single configuration can map Source → Target and Target → Source, including unflattening of previously flattened members.

AutoMapper provides this via ReverseMap. Mapperly requires separate methods; Mappa should define an explicit, compile-time approach.

Current behavior

Mappa generates one-way maps only. There is no mechanism to derive or declare a reverse map from an existing partial method.

Proposed scope

• Attribute or convention to declare a reverse companion map (or auto-generate when safe).
• Reverse property/constructor parameter pairing consistent with forward map overrides (MappaUseProperty, etc.).
• Integration with flattening/unflattening (#120) when nested paths are involved.
• Clear diagnostics when reverse mapping is ambiguous or unsupported.

Acceptance criteria

• Reverse/bidirectional maps can be declared and generated
• Happy-path and negative-path generator tests
• Documentation and samples
• All tests pass via .\Scripts\RunTestsAndReportCoverage.ps1

Workflow notes

• Plan file: .cursor/plans/issue_<N>_reverse_bidirectional_mapping.plan.md
• Related to #120 for unflattening