StakeStakeV2 zero copy api [wincode]

[grod220]

Adds a new zero‑copy state API backed by wincode.

Key Changes

• Adds p-stake-interface crate (#![no_std] compatible)
• Introduces alignment-1 POD types (PodU32, PodU64, PodI64, PodAddress) for safe zero-copy from
  unaligned slices
• New API via StakeStateV2:
  • StakeStateV2::from_bytes(&[u8]) -> Result<&StakeStateV2, StakeStateError> for read-only access
  • StakeStateV2::from_bytes_mut(&mut [u8]) -> Result<&mut StakeStateV2, StakeStateError> for
    mutations
  • Tag-checked accessors: meta(), stake(), meta_mut(), stake_mut()
  • State transitions: initialize(), delegate()
• Maintains full ABI compatibility with legacy bincode-encoded stake accounts

Motivation

Deserialization via bincode/borsh is computationally expensive for programs that only need to access specific fields or verify the state. This zero-copy approach allows for significantly lower compute unit (CU) usage when interacting with stake accounts.

[febo]

This could be zero-copy if you do a transmute instead of StakeStateV2Tag::deserialize, but not sure if there is any benefit in doing that.

[grod220]

Would we lose some safety though? What do you think about this instead?

    pub fn from_bytes(bytes: &[u8]) -> Result<Self, StakeStateError> {
        if bytes.len() < 4 {
            return Err(StakeStateError::UnexpectedEof);
        }
        let raw = u32::from_le_bytes(bytes[..4].try_into().unwrap());
        Self::from_u32(raw)
    }

[febo]

We should add wincode support to Address, then this type is not needed.

[febo]

Since this is a breaking change, could we stop using the enum? Seems like all we need is already in StakeStateV2Layout, so that one could become the state "account". One benefit is that we won't need to have the *View and *Write type. The fields not applicable for a variant can be optional.

[grod220]

(Discussed offline)

Going to revise the API, but ensure a private-field pattern to ensure only methods can make state transitions to prevent invalidate state.