feat(riscv-guest): l2 exec guest coverage test harness

[Filter94]

This PR implements issue(s) #

Checklist

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• If this change is deployed to any environment (including Devnet), E2E test coverage exists or is included in this
  PR.
• I have informed the team of any breaking changes if there are any.

[Copilot]

Pull request overview

Adds a host-side test harness for the l2-execution guest by introducing a test-only SSZ encoder for vanilla StatelessInput, a conflation-plan DSL + scenario suite, and an execution seam that allows exercising conflation logic end-to-end with a stubbed per-block engine.

Changes:

• Add a test-only SSZ encoder (stateless_input_encode) plus round-trip and golden re-encode tests against EF fixtures.
• Introduce a ConflationPlan DSL, parity tests for its StubEngine, and a multi-scenario range test suite for conflation invariants.
• Refactor guest entry to runL2ExecutionWithEngine and tighten non-genesis parent-header witness requirements in per-block execution.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
riscv-guests/l2-execution/test/stateless_input_encode.zig	New test-only SSZ encoder for vanilla StatelessInput wire bytes.
riscv-guests/l2-execution/test/stateless_input_encode_test.zig	Round-trip and golden-byte tests for the new encoder.
riscv-guests/l2-execution/test/legacy_tx_rlp.zig	Shared helper to RLP-encode legacy transactions for fixtures/tests.
riscv-guests/l2-execution/test/l2_execution_range_test.zig	End-to-end conflation scenario suite (happy path + single-mutation rejects).
riscv-guests/l2-execution/test/conflation_plan.zig	New DSL to fabricate consistent multi-block inputs and drive conflation via a stub engine.
riscv-guests/l2-execution/test/conflation_plan_parity_test.zig	Guards stub realism against the real execution seam and smoke-tests the DSL.
riscv-guests/l2-execution/src/l2_execution.zig	Introduce runL2ExecutionWithEngine seam and use it from runL2Execution.
riscv-guests/l2-execution/src/execution.zig	Require resolvable parent header for non-genesis blocks (avoid self-referential pre-state).
riscv-guests/l2-execution/build.zig.zon	Add ssz library dependency for test-side serialization.
riscv-guests/l2-execution/build.zig	Wire new test modules/tests and add ssz import for the encoder module.

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