fix(key-store): reject accounts sharing master privacy keys

[nchamo]

Summary

• Stopgap for PrivateMutable initialization nullifier shared between two accounts with same keys but different addresses (notes not shared) #24821: aztec-nr derives PrivateMutable/PrivateImmutable initialization nullifiers from the nullifier-hiding key and the storage slot, without the owner address, so two accounts sharing privacy keys silently corrupt each other's state variables.
• KeyStore.addAccount now throws when a different address is already registered with any of the four master privacy keys, since only the key holder can trigger the collision, making PXE registration the right choke point. Re-registering the same account stays idempotent.
• E2e suites that reused one secret across addresses out of convenience (crowdfunding decoy escrow, account-contract guide, bot bridge-resume) now use distinct secrets. The multiple_accounts_1_enc_key suite tests the now-rejected scenario itself, so it is skipped until the restriction is lifted (tracked in F-835).
• To be removed once the Noir fix lands. Note a user registering the two accounts in separate PXEs can still hit the issue.

[nchamo]

Decided against this