feat: add belongs-to-many pivot models

[elpete]

Review

This extracts and expands the pivot-table behavior that was previously bundled into #331. It is the first PR in the stack; #331 will contain only relationship create() support after this PR lands.

Eloquent's belongs-to-many model is the reference point: related models always carry an intermediate model containing both pivot keys, withPivot() selects additional columns, as() renames the accessor, using() selects a custom model, and the relationship owns pivot-specific query and write helpers. Reference: https://laravel.com/docs/12.x/eloquent-relationships#retrieving-intermediate-table-columns

Quick semantics and choices

• Every BelongsToMany result receives a loaded pivot relationship containing both pivot keys. withPivot() adds declared columns.
• as() renames that loaded relationship, so normal Quick access such as getSubscription() works.
• The default Pivot@quick model is read-only. Its schema is assembled at runtime, so allowing save() would imply persistence guarantees Quick cannot safely make.
• using( "PostTag" ) opts into a declared Pivot entity. It supports normal Quick properties, aliases, casts, methods, lifecycle hooks, and explicit save(); it must extend quick.models.Relationships.Pivot.
• Pivot models retain their parent and related entities through getPivotParent() and getPivotRelated(), without introducing cascading persistence.
• Pivot keys cannot be overwritten by extra attachment/update data.
• withTimestamps() accepts explicit created/modified column names because Quick applications do not share one universal timestamp convention.
• Multi-ID attach() and sync() intentionally apply one pivot-attribute struct to every row. Per-ID attribute maps are not introduced in this PR.

API

• withPivot()
• as()
• using()
• withTimestamps()
• wherePivot(), orWherePivot()
• wherePivotIn(), wherePivotNotIn()
• wherePivotBetween(), wherePivotNotBetween()
• wherePivotNull(), wherePivotNotNull()
• orderByPivot(), orderByPivotDesc()
• withPivotValue()
• pivot attributes on attach() and sync()
• updateExistingPivot()

Test-first evidence

Before implementation, seven public relationship-flow specs errored because withPivot(), using(), and withTimestamps() did not exist. The tests exercise lazy and eager loading, the default and custom accessors, a custom Pivot cast and method, explicit custom-Pivot persistence, the read-only default model, pivot constraints/order, attachment attributes, updates, defaults, and timestamps.

Validation

• Focused belongs-to-many suite: 11 passed, 0 failed, 0 errors
• Full suite on Lucee 6: 566 passed, 0 failed, 0 errors, 3 skipped
• Formatter completed
• git diff --check passed

Supports #84.

[elpete]

CI follow-up: the first matrix exposed three portability details in the new pivot hydration path—Adobe property declaration ordering, Adobe full-null handling for an unset custom pivot mapping, and BoxLang representing the declared-but-unset mapping as null. Those paths now use engine-neutral component ordering and an explicit empty-string default. The final matrix is green across all 27 checks.