Enforce a non-empty FilePair invariant

[woutervanranst]

Problem statement

internal sealed record FilePair
{
    /// <summary>
    /// The relative path to the BinaryFile
    /// </summary>
    public required RelativePath RelativePath { get; init; }

    public BinaryFile? Binary { get; init; }

    public PointerFile? Pointer { get; init; }
}

Line 20-22 currently allows Binary and Pointer to both be null, which creates an invalid archive item state. Please fail fast at creation (constructor/factory guard) so invalid pairs cannot enter hashing/upload flow.

That creates duplicate path state inside the same model:

• for binary-only pairs, RelativePath mirrors BinaryFile.Path
• for pointer-only pairs, RelativePath is the logical binary path while PointerFile.Path is the physical pointer-file path
• for binary-plus-pointer pairs, the relationship between all three paths is implied rather than enforced by the type itself

This duplication makes the canonical path an explicit field instead of a derived domain concept, and it leaves room for inconsistent states if future callers construct mismatched values.

Improvement

Evaluate whether FilePair.RelativePath should become a derived property instead of stored state.

Possible direction:

• infer the canonical repository path from BinaryFile.Path when a binary exists
• otherwise infer it from PointerFile.Path.ToBinaryPath()
• change factories to accept only the component models:
  • FromBinary(BinaryFile binary)
  • FromPointer(PointerFile pointer)
  • FromBinaryWithPointer(BinaryFile binary, PointerFile pointer)
• validate in FromBinaryWithPointer(...) that the pointer resolves back to the binary path

Future enhancement

This is a cleanup/design improvement rather than a required fix.

If implemented later, it should:

• preserve the current logical path behavior for pointer-only pairs
• keep call sites simple in the archive pipeline
• add tests that prove mismatched binary/pointer paths are rejected
• confirm whether any downstream code still needs access to the physical pointer-file path separately from the canonical repository path