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

This PR significantly expands the riscv-guests/l2-execution test and tooling surface to improve coverage and debuggability of the extended L2 execution guest, including (1) a test-only SSZ encoder for vanilla stateless inputs, (2) a conflation-plan DSL with scenario/parity tests, and (3) native host tools and JSON output support for local runs.

Changes:

• Added a test-only SSZ encoder for vanilla StatelessInput plus round-trip and golden re-encode tests against EF fixtures.
• Added a runL2ExecutionWithEngine seam and a conflation-plan DSL + scenario/parity tests to exercise conflation logic end-to-end on real bytes with a stubbed per-block engine.
• Added native host tools (l2-execution-wrap, l2-execution-runner) and a JSON output encoder + golden test; wired new tests/tools/deps into build.zig.

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
riscv-guests/l2-execution/test/stateless_input_encode.zig	Test-only SSZ encoder for vanilla StatelessInput wire format.
riscv-guests/l2-execution/test/stateless_input_encode_test.zig	Round-trip + golden re-encode tests validating the encoder against the real decoder/fixtures.
riscv-guests/l2-execution/test/legacy_tx_rlp.zig	Shared legacy transaction RLP builder for test fixtures.
riscv-guests/l2-execution/test/l2_execution_wrap.zig	Native CLI tool to wrap vanilla inputs into extended inputs for the extended guest/harness.
riscv-guests/l2-execution/test/l2_execution_ssz_test.zig	Unit tests for SSZ input/output encoding/decoding behavior.
riscv-guests/l2-execution/test/l2_execution_runner.zig	Native CLI runner for extended guest logic with SSZ/JSON output toggle.
riscv-guests/l2-execution/test/l2_execution_range_test.zig	Scenario suite exercising conflation logic over a realistic multi-block range plus mutation cases.
riscv-guests/l2-execution/test/l2_execution_json.zig	JSON encoder for guest output (native-only convenience).
riscv-guests/l2-execution/test/l2_execution_json_test.zig	Golden JSON shape test matching the Python reference output (minus proof metadata).
riscv-guests/l2-execution/test/conflation_plan.zig	Conflation-plan DSL + stub engine to generate consistent inputs and drive conflation logic.
riscv-guests/l2-execution/test/conflation_plan_parity_test.zig	Guards stub realism vs real execution seam + DSL smoke tests.
riscv-guests/l2-execution/src/l2_execution.zig	Adds runL2ExecutionWithEngine seam and exposes internal helpers for tests via test_api.
riscv-guests/l2-execution/src/execution.zig	Tightens pre-state root derivation: requires resolvable parent header witness for non-genesis.
riscv-guests/l2-execution/build.zig.zon	Adds ssz.zig dependency (lazy) for test-only SSZ serialization.
riscv-guests/l2-execution/build.zig	Wires in new tests/modules and installs new native tools.

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