Final answer:
Lady Macbeth's analogy for keeping her word to Macbeth is not stated in the given excerpts, but her firm and unwavering demeanor implies a comparison to a rock-solid contract.
Step-by-step explanation:
The analogy Lady Macbeth uses to demonstrate her commitment to keeping her word to Macbeth is not explicitly mentioned in the text you provided. However, based on the character's overall dialogue and actions from Shakespeare's Macbeth, one could infer that Lady Macbeth's assurance of her resolve might be comparable to a rock-solid contract given her manipulative and determined nature to see their plan through.