add ContractExecutor dispatch enum (Aot + Emu)

[avi-starkware]

Summary

Adds a public dispatch enum so a single call site can pick between cairo-native's AOT executor and the sierra-emu interpreter at runtime, without forcing every caller to maintain its own match.

Replaces #1598 / #1608. The new version drops SierraEmuSyscallBridge entirely — the prior PR in this stack (extracting cairo-native-syscalls) made cairo-native's and sierra-emu's StarknetSyscallHandler the same trait, so the handler flows through both paths unchanged.

Changes

• src/executor/contract_executor.rs (new):
  • pub enum ContractExecutor { Aot(AotContractExecutor), Emu(EmuContractInfo) }, with Emu gated on the new sierra-emu cargo feature.
  • pub struct EmuContractInfo { program: Arc<Program>, entry_points: …, sierra_version: … }. The Arc<Program> is shared across invocations rather than cloned per call.
  • From impls for both variants.
  • ContractExecutor::run dispatches:
    • Aot arm → AotContractExecutor::run.
    • Emu arm → sierra_emu::VirtualMachine::new_starknet(…).run(&mut syscall_handler). No bridge wrapping the handler.
  • convert_builtin_costs translates cairo_native::utils::BuiltinCosts → sierra_emu::BuiltinCosts (these types are still separate; only the syscall handler shape converged).
• src/executor.rs: register the new module and re-export ContractExecutor (+ EmuContractInfo under the sierra-emu feature).
• Cargo.toml:
  • new sierra-emu = ["dep:sierra-emu"] feature.
  • with-trace-dump now depends on sierra-emu rather than the optional dep directly, so the two paths agree.

Diff against the original #1608

Aspect	#1608 (closed)	this PR
Handler in Emu arm	wrapped in SierraEmuSyscallBridge	passed directly
Bridge file	required	doesn't exist
Type conversions	convert_u256 / convert_secp_*_point / convert_execution_info{,_v2}	none (types are identical via re-export)

Stack

1. align sierra-emu StarknetSyscallHandler trait/types with cairo-native #1610 — trait alignment
2. extract shared cairo-native-syscalls crate #1611 — extract shared crate
3. this PR — ContractExecutor (supersedes add ContractExecutor dispatch enum (Aot + Emu) #1598 / add ContractExecutor dispatch enum (Aot + Emu) #1608)
4. (next) run_with_libfunc_profile + AotWithProgram (supersedes add run_with_libfunc_profile + AotWithProgram variant for ContractExecutor #1599 / add run_with_libfunc_profile + AotWithProgram variant for ContractExecutor #1609)

Test plan

• cargo check (default features) clean
• cargo check --features sierra-emu clean
• cargo check --features with-trace-dump clean (transitively activates sierra-emu)
• cargo check --workspace --all-features clean
• CI green

[ghost]

✅ Code is now correctly formatted.