deps: upgrade snarkvm to v4.8.1 (rehearsal)

[iamalwaysuncomfortable]

REHEARSAL PR — produced by a dry-run of the /upgrade-snarkvm automation (pin was staged back to v4.8.0 first). Review the shape, then close without merging.

Changeset (v4.8.0 → v4.8.1)

• ConsensusVersion::V17 introduced: anchor time reverts to 25s; mainnet heights set for V16 (19,860,000) and V17 (19,860,001); test-heights table extended to V17→20.
• Network::ANCHOR_TIMES const grew from 2 to 3 entries (type change; SDK does not reference it directly).
• History indexing performance fix in ledger/store internals; cargo-audit dep bump.

Adaptation needed: none — sdk/src/programs/process.rs already targets ConsensusVersion::V17 (the code was adapted when v4.8.1 originally landed; this rehearsal only rolled back the pin).

Dependency form

• sdk/Cargo.toml: switched to crates.io form (snarkvm = { version = "4.8.1", ... }) — 4.8.1 is published on crates.io. Features preserved verbatim.
• sdk-abi/Cargo.toml: leo HEAD (60d258d) now pins snarkvm via crates.io version = "4.8.1", so sdk-abi mirrors that line exactly and bumps all leo revs to 60d258d (type-coupling with leo-disassembler preserved).

Validation

• cargo fmt --check, cargo clippy (mainnet, -D warnings), cargo check (testnet): clean
• Both-network wheel build + merge: OK
• pytest fast suite: 874 passed
• Proving suite: 4 passed, 1 skipped; testnet surface: 6 passed
• aleo-contract-abi-generator suite: 8 passed; abi hook: 3 passed
• Build-graph isolation canary (dev_skip_checks not in sdk/Cargo.lock): clean

Version bumps (one patch above live PyPI, 0.3.0)

• aleo-sdk 0.3.1, aleo-contract-abi-generator 0.3.1, shield-swap-sdk 0.3.1