feat: Add offset option to has_one and has_many relationships

[nallwhy]

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• Bug fixes include regression tests
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• Documentation changes
• Features include unit/acceptance tests
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Summary

• Add offset option to has_one and has_many relationship DSL
• offset on has_one implies from_many?: true to ensure lateral joins

Closes #1740

[Copilot]

Pull request overview

Adds first-class offset support to relationship DSL for has_one and has_many, enabling “nth record”/sliced relationship loads without manual relationships. For has_one, using offset implicitly switches to from_many?: true so the loader uses the correct lateral-join behavior.

Changes:

• Add offset option to Ash.Resource.Relationships.HasOne and HasMany DSL/entities.
• Apply relationship-level offset during relationship query construction/loading.
• Add unit/behavior tests covering offset (and limit slicing) plus formatter support for offset 1 DSL style.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/ash/resource/relationships/has_one.ex	Adds :offset field/DSL option; offset implies from_many?: true.
lib/ash/resource/relationships/has_many.ex	Adds :offset field/DSL option to has_many.
lib/ash/actions/read/relationships.ex	Applies relationship offset when building relationship queries.
test/resource/relationships/has_one_test.exs	Tests has_one DSL representation includes offset and implied from_many?.
test/resource/relationships/has_many_test.exs	Tests has_many DSL representation includes offset and limit.
test/actions/has_many_test.exs	End-to-end load tests verifying has_one offset selection and has_many offset+limit slicing.
.formatter.exs	Allows offset 1 without parentheses in formatted DSL blocks.
.github/CONTRIBUTING.md	Fixes numbering in DSL documentation improvement checklist.

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[zachdaniel]

@nallwhy this looks good, but we will also need to add this option to ash_sql for joining and for ash_postgres in its lateral join code.