ORM: Pluck and Chunk helpers

[devituz]

Goal

Add Pluck (fetch a single column into a slice) and Chunk (cursor-friendly batched fetch) to orm.Query[T]. Mirrors Eloquent's APIs.

Acceptance criteria

• Pluck[V any](ctx, query, col string) ([]V, error) returns a typed slice
• Chunk[T any](ctx, query, size int, fn func([]T) error) error streams batches
• Both use a single SQL statement per batch (no per-row queries)
• Tests against SQLite covering empty, partial-last-batch, and error-mid-stream cases