feat!: remove DEFAULT_CIPHER/DEFAULT_NONCE public constants

[iduartgomez]

Problem

DelegateRequest::DEFAULT_CIPHER ([u8; 32]) and DelegateRequest::DEFAULT_NONCE ([u8; 24]) seeded a world-known XChaCha20-Poly1305 key and a fixed 24-byte nonce. Any freenet node that started without explicit --cipher / --nonce configuration encrypted all delegate secrets under this pair. Combined with the per-delegate nonce reuse fixed in freenet-core PR #4143, the result was that default-configured nodes' delegate secrets were trivially recoverable by anyone who could read the secrets directory.

PR #4143 closed the catastrophic-reuse half (per-write random nonce + on-disk versioned format with legacy fallback). This stdlib PR closes the world-known-key half so the next freenet-core PR can replace the DEFAULT_CIPHER fallback with auto-generated, per-node persisted ciphers.

Solution

Delete the two pub const declarations from DelegateRequest. The wire format RegisterDelegate { cipher: [u8; 32], nonce: [u8; 24] } is unchanged — only the named public constants are removed. Servers running freenet-core >= 0.2.59 generate per-write random nonces and treat the registration nonce as a read-side legacy fallback only.

Callers that previously named the consts must now:

• Generate a fresh 32-byte cipher per delegate (e.g. XChaCha20Poly1305::generate_key(&mut OsRng)).
• Pass any 24-byte nonce value for the wire field (commonly [0u8; 24]); it is no longer used by the server for new writes.

Version bump

0.7.0 → 0.8.0. Pre-1.0 semver: removing public consts is a breaking change for any consumer that named them, so the minor version bumps.

Wire-format pin tests

Untouched. The enum variant discriminants for DelegateRequest are not reordered, and the field shape of RegisterDelegate is unchanged. *_wire_format_is_stable tests continue to pass without modification.

Testing

cargo test --all — 19 passed, 0 failed; doc-tests pass.
cargo fmt --all --check clean.
cargo clippy --all -- -D warnings clean.

Next

After merge + cargo publish 0.8.0 to crates.io, freenet-core PR B2 will:

• Bump freenet-stdlib = "0.8.0".
• Replace the DEFAULT_CIPHER fallback in crates/core/src/config/secret.rs::SecretArgs::build with an auto-generated cipher persisted to secrets_dir/delegate_cipher (mirroring the existing transport_keypair auto-persist).
• Drop the --nonce CLI flag (nonces are now per-write).
• Update test/fdev call sites that named the removed consts.

Refs: freenet/freenet-core#4137 (tracker), freenet/freenet-core#4139 (sub-issue).