Add Automatic Nested Object Mapping (Embedded Maps)

[j-d-ha]

Priority: P1 - Critical
Tier: 2 - Single-Table Support
Effort: Large (12-20 hours)

Already In Codebase

• No nested-object mapping exists today. The generator only supports scalar/enum-like mappings (plus custom methods).
• The generated FromItem currently uses an object initializer: src/LayeredCraft.DynamoMapper.Generators/Templates/Mapper.scriban.

Goal

Support automatic recursive mapping of complex CLR types as embedded DynamoDB maps (AttributeValue.M).

This story is intentionally separate from collection mapping (lists/maps/sets).

Scope

• If a property type is not a supported scalar/enum and is not a supported collection shape, treat it as a nested object.
• Serialize nested objects to AttributeValue { M = ... }.
• Deserialize nested objects from AttributeValue.M.

Implementation notes

• Implement recursion in the generator: for each nested complex type encountered, generate mapping code for its members using the same rules.
• Ensure naming convention conversion is applied at each level (DynamoNamingConvention conversion already exists).
• Respect [DynamoIgnore] and [DynamoField(AttributeName=...)] at each mapped type.
• Add cycle detection:
  • If type graph contains a cycle (A -> B -> A), emit a diagnostic and do not generate mapping.

Diagnostics

• Unsupported nested member type
• Missing map (M) when expected
• Cycle detected in nested object graph

Tests

Add verify tests in test/LayeredCraft.DynamoMapper.Generators.Tests/ covering:

• One-level nesting
• Multi-level nesting
• Nesting + naming convention changes
• Nesting + ignore/field overrides
• Cycle detection diagnostic

Acceptance criteria

• Nested complex properties serialize/deserialize as embedded maps.
• Overrides (convention, ignore, field name override) are honored inside nested objects.
• Cycles are detected and reported via diagnostics.
• Snapshot tests cover positive and negative scenarios.