runar-rs: add CallOptions.locktime to enable deadline-based contracts

[E-Jacko]

Fixes #40.

Summary

Adds an optional locktime: Option<u32> field to CallOptions (and the lower-level CallTxOptions) in packages/runar-rs/. Defaults to None. When set, the SDK uses the provided value as the call transaction's nLockTime; when unset, the SDK falls back to the legacy hardcoded 0.

Unblocks contracts that assert against extractLocktime(txPreimage) (deadline-based auctions, escrow timeouts, vesting releases, etc.).

Changes

• packages/runar-rs/src/sdk/types.rs — add locktime: Option<u32> field to CallOptions
• packages/runar-rs/src/sdk/calling.rs — add matching field to CallTxOptions + read options.and_then(|o| o.locktime).unwrap_or(0) at the non-terminal call-tx build site
• packages/runar-rs/src/sdk/contract.rs — thread CallOptions.locktime through to both invocations of build_call_transaction_ext (initial build + rebuild path) and into the terminal-path's build_terminal_tx closure

Backwards compatibility

Purely additive. Existing callers that don't set locktime see byte-identical output. All 307 lib tests in packages/runar-rs/ pass unchanged.

Minor API note: callers that construct CallOptions field-by-field without ..Default::default() will see a missing-field compile error. Suggest documenting in the changelog. Happy to add #[non_exhaustive] if maintainers prefer (itself a breaking change but future-proofs the struct).

Test plan

• cargo test --release --lib in packages/runar-rs/ — 307/307 passes
• cargo check clean

A regression test suite covering default + override behavior is in flight as a follow-up commit if helpful — happy to include in this PR or land separately.

Mainnet validation

The same patch (vendored locally against the same code paths) has been validated end-to-end on BSV mainnet. A test EnglishAuction was deployed and bid/closed using a non-zero deadline:

• Bid (state-continuing — regression evidence): 09814d16…6244c block 949122
• Close (terminal, deadline-asserting — primary evidence): 779ea679…3606 block 949222

Without the locktime override, the close transaction is rejected with Script evaluated without error but finished with a false/empty top stack element (the script's deadline assertion failing). With this PR, the same transaction mines.

Open design question

Should the default be the current chain tip rather than 0? 0 preserves legacy behavior (back-compat safe). Chain-tip default would be more "useful out of the box" but is a behavior change. Happy to discuss in review.

Related

This is the first of three fixes that together enable auction-style terminal-method contracts to mine on mainnet:

1. This PR — nLockTime override (deadline-assertion preconditions)
2. Companion issue/PR — terminal-method sighash subscript trim (BIP-143 compliance)
3. Companion issue/PR — compiler stack-cleanup gate (cross-compiler, see issue for scope)

Issues 2 and 3 will follow with their own PRs.