feat(discovery-core): add secretfelt type and unify key param naming

[m-kus]

TL;DR

Implemented secure key handling with SecretFelt to protect private keys from memory exposure.

What changed?

• Added a new SecretFelt wrapper type that automatically zeroes memory when dropped
• Updated all functions that handle private keys to use SecretFelt instead of raw Felt
• Added implementation of decrypt_outgoing_recipient_addr function
• Added zeroize dependency for secure memory handling
• Moved serde from dev-dependencies to regular dependencies

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This change is 

[m-kus]

• feat(discovery-core): add transfer preflight check #474
• feat(discovery-core): add outgoing state sync orchestrator #473
• feat(discovery-core): add incoming state sync orchestrator #472
• feat(discovery-core): add paginated last-note-index search with bisection #471
• feat(discovery-core): add paginated outgoing channel discovery #470
• feat(discovery-core): paginated notes discovery with nullifier filtering #462
• feat(discovery-core): paginated subchannel discovery with cursor support #461
• feat(discovery-core): paginated incoming channel discovery with cursor support #460
• feat(discovery-core): add iobudget batching, cursor types, and new cost constants #459
• feat(discovery-core): add secretfelt type and unify key param naming #458 👈 (View in Graphite)
• feat(discovery-core): add batch iviews methods and outgoing channel support #457
• feat(discovery-core): add hash primitives and outgoing channel storage slots #456
• chore(claude): move claude.md into .claude and split rules into individual files #455
• refactor(indexer): move devnet dump generation to a separate script, scaffold e2e tests dir #454
• fix(sdk): sync sdk with cairo contract renames (to_addr, enc_user_addr_hash) #469
• main

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[Yoni-Starkware]

@Yoni-Starkware reviewed 8 files and all commit messages, and made 3 comments.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @m-kus).

---

crates/discovery-core/src/privacy_pool/hashes.rs line 129 at r1 (raw file):

    token: Felt,
    index: u64,
    private_key: &super::types::SecretFelt,

Move the use to the top of the file; avoid using super in non-test context

Suggestion:

 private_key: &SecretFelt,

---

crates/discovery-core/src/test_fixtures.rs line 144 at r1 (raw file):

    };
    use crate::io_budget::IoBudget;
    use crate::privacy_pool::types::SecretFelt;

Please move up

Code quote:

    use crate::io_budget::IoBudget;
    use crate::privacy_pool::types::SecretFelt;

[m-kus]

@m-kus resolved 2 discussions.
Reviewable status: 6 of 9 files reviewed, all discussions resolved (waiting on @Yoni-Starkware).