feat: add update() for bulk attribute updates

[Artmann]

Summary

Adds an update(attributes) instance method to BaseModel so multiple attributes can be updated in one call instead of assigning fields one by one and calling save():

const product = await Product.find(id)

await product.update({ name: 'Chair 1', price: 42.0 })

• Implemented as Object.assign(this, attributes) + save(), so all timestamp stamping and upsert behavior stays in one place — behaviorally identical to manual assign-then-save.
• Typed as Partial<this>, matching the existing create(attributes: Partial<T>) convention; unknown keys and wrong value types are compile errors.
• No fill() method: without a mass-assignment guard concept it would just be Object.assign, which users can already write.

Changes

• packages/esix/src/base-model.ts — new update() method after save(), with JSDoc.
• packages/esix/src/base-model.spec.ts — unit tests: existing-model update (merged $set, stamped updatedAt, upsert) and no-id insert (createdAt, wasRecentlyCreated).
• packages/esix/src/integration-test.spec.ts — end-to-end test against the mock adapter.
• packages/website/docs/inserting-and-updating-models.md — "Updating multiple attributes" docs section.

Testing

• yarn lint, yarn typecheck, yarn test all green (238 tests, +3 new).
• Reviewed by a fresh-context review pass before opening this PR.

Closes #44

🤖 Generated with Claude Code

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.