feat: add whereBelongsTo relationship constraints

[elpete]

Closes #155

Issue review

Recommendation: 9/10. whereBelongsTo is a strong fit for Quick. It expresses a common foreign-key filter in relationship terms, avoids duplicating key and column knowledge at call sites, and follows an established ORM convention.

Reasons for:

• Callers can pass a loaded parent entity instead of manually extracting its key and remembering the child foreign key.
• The relationship definition remains the source of truth for aliases, custom keys, and composite keys.
• Supporting arrays makes the API useful for bulk parent selections without manual tuple construction.
• The method composes naturally with existing Quick query methods and has an orWhereBelongsTo counterpart.

Reasons against / tradeoffs:

• Optional relationship-name inference only works for conventional names based on the related entity name; aliases such as author must be explicit.
• Accepting entities and collections adds validation and grouping logic beyond a simple single-key shortcut.
• An empty related collection is treated as invalid input rather than silently returning no rows, making accidental empty filters visible.

Test-first result

Five public integration tests were written before implementation for:

• an explicitly named aliased relationship;
• conventional relationship-name inference;
• multiple related entities;
• composite keys;
• OR composition.

Before implementation, the focused bundle had 37 passes and 4 errors because whereBelongsTo was missing. The OR case was added after the core method and validates its companion API.

Implementation

• Add whereBelongsTo( related, relationshipName?, combinator = "and" ) to QuickQB.
• Accept one Quick entity, an array, or a collection exposing get().
• Resolve and validate the belongs-to relationship without applying instance constraints.
• Build grouped OR tuples from the relationship foreign/local key pairs.
• Add orWhereBelongsTo() as the fluent OR counterpart.
• Reject empty inputs, mixed entity mappings, and non-belongs-to relationships with focused Quick exceptions.

Validation

• Focused QueryingRelationshipsSpec: 42 passed, 0 failed, 0 errors
• Full suite: 500 passed, 0 failed, 0 errors, 3 skipped
• box run-script format
• git diff --check

Dependency: latest prerelease qb 14.0.0-beta.3.