feat(relations): withPivotValue for belongsToMany

[3m1n3nc3]

Adds withPivotValue(column, value) and withPivotValue({ column: value }) to belongsToMany relationships. It:

• constrains the relationship to pivot rows where the column equals the value (via wherePivot, so it applies to reads, eager loads, and mutation scoping), and
• records the value as the default for that column when a new pivot row is created through the relationship (attach/create/save/sync). Explicit pivot attributes still override it, and the foreign/related keys always win.

Tests: default on create/attach, object form, attribute override, and the
read-side filter. Docs: pivot helpers section.