Rename `seed` to `id_salt` to avoid confusion with wallet **seed** phrases

The term `seed` in Miden refers to a value used during account ID generation. This is fundamentally different from how "seed" is used in Ethereum/Bitcoin, where it refers to BIP39 mnemonic phrases for hierarchical key derivation and wallet recovery.

This terminology overlap is confusing for developers coming from the Ethereum ecosystem.

I (re-)realized this when reviewing https://github.com/0xMiden/miden-base/pull/1805/, and there the `AccountDataExport` actually includes "seed", which is counterintuitive.

### Proposal

Rename `seed` -> `account_salt`

Alternatives:
- `id_salt` - shorter but less explicit, might also be confusing
- `account_id_salt` (my second favorite)
- `account_genesis_salt` - emphasizes one-time use, but maybe too verbose?