Add convention-based customization to the Sep source generator

[Copilot]

The source generator previously required attribute-configured converters for custom parsing and formatting. This adds compile-time conventions that emit direct, allocation-free calls while retaining converter compatibility.

Changes

• Member conventions

  • Discover Parse{Member}, TryParse{Member}, and Format{Member} on partial adapters.
  • Support SepReader.Col/SepWriter.Col and row-level hooks for computed or multi-column values.
  • Preserve nullable types and generated optional-column handling.

• Type-wide conventions

  • Apply Parse, TryParse, and Format overloads to members with matching declared types.
  • Resolve row before column hooks and member-specific before type-wide hooks.

• Diagnostics

  • Report invalid or ambiguous signatures, incomplete parse pairs, and converter conflicts.
  • Make TryParse-only hooks throw an informative FormatException from generated Parse.

• Compatibility

  • Retain SepCol.Converter as a deprecated fallback.
  • Document the convention model and migration path.

[SepSourceGeneration(typeof(Person))]
public static partial class PersonSepExtensions
{
    static StrongId ParseId(SepReader.Col col) => new(col.Parse<int>());

    static bool TryParseId(SepReader.Col col, out StrongId value)
    {
        var success = col.TryParse<int>(out var parsed);
        value = new(parsed);
        return success;
    }

    static void FormatId(SepWriter.Col col, StrongId value) =>
        col.Format(value.Value);
}

[nietras]

@copilot there needs to be convention based overriding of which column index or name is used based on header as parameter or null if none. think this through. and remove old converter based stuff.

[Copilot]

@copilot there needs to be convention based overriding of which column index or name is used based on header as parameter or null if none. think this through. and remove old converter based stuff.

Implemented in ea00238 with follow-up validation in 9de3f22. GetColumn{Member}(SepReaderHeader?) can now select a read column by index or name (with null for headerless readers), and converter-based configuration has been removed.