docs(postmortem): add account-factory-salt-collision.md

[Glam26]

Here is what was done:

1. Created a new branch named postmortem-account-factory-salt-collision.
2. Created the file account-factory-salt-collision.md.
3. Drafted the document addressing all your requirements:
   • Explained deterministic address derivation simply (e.g., CREATE2, the
   formula, and salt's role).
   • Walked through a concrete worked example with two separate
   overlapping batch_initialize calls.
   • Added general guidance on salt design for future programmatic
   contract deployments.
4. Committed the newly added postmortem document.

Feel free to review the file and let me know if you need any adjustments to
the wording or structure!

closes #310