feat(echo-venue): settle instantly, add echo-client round trip

[mfw78]

What

• echo-venue: status() now returns IntentStatus::Settled(None) instead of IntentStatus::Open, so the adapter settles instantly on first poll.
• echo-venue: adds a host #[cfg(test)] conformance module that bridges the macro-minted bindgen IntentHeader to nexum-venue-test's GoldenHeader mirror, holding derive_header to a hand-written golden (derive_header_conforms_to_the_published_golden), plus a divergence test with a little-endian amount (divergent_derivation_is_caught_by_the_golden) proving the check rejects a wrong derivation.
• Adds echo-client (modules/examples/echo-client), the strategy half of the echo pair: declares required = ["pool", "logging"] in module.toml, submits an opaque body through nexum:intent/pool to echo-venue on every chain-1 block, and logs the intent-status transitions the router fans back.
• Adds e2e_echo_module_router_adapter_round_trip in crates/nexum-runtime/src/supervisor/tests.rs: boots the supervisor with the echo-venue adapter and echo-client module, dispatches a chain-1 block, polls the pool router's status transitions, asserts the delivered status is Settled, and asserts the module's own logs record both the submission and the received settled status.
• justfile: adds a build-echo-client target; test-e2e now also builds echo-venue and echo-client before running.
• .github/workflows/ci.yml: adds echo-client to the wasm build matrix alongside echo-venue.
• Cargo.toml / Cargo.lock: registers the new modules/examples/echo-client workspace member.

Why

Implements the first-train cut line, proving the intent core end to end with real components on both sides of the router: a real module submits through the host router to a real venue adapter and gets the settled status routed back, with no scripted stand-ins.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings (scoped: cargo clippy -p echo-venue --target wasm32-wasip2 --all-targets --locked, clean)
• cargo test --workspace (scoped echo-venue/echo-client wasm build plus host-side tests; the e2e round-trip test run twice, plus the full 11-test e2e suite)
• Confirmed the round-trip test has teeth: reverting echo-venue's status to Open and rebuilding the wasm makes e2e_echo_module_router_adapter_round_trip fail; restoring the fix makes it pass again.

AI Assistance

Claude (Opus) used for the implementation.

Closes #234

[jean-neiverth]

minor issue: modules/examples/echo-client/src/lib.rs, on_intent_status - logs the receipt length but not the status value (Settled, Open, etc.). The e2e test assertion m.contains("intent status from venue echo-venue") matches regardless of the actual settlement state. Logging update.status would strengthen both human observability and the test assertion that the module observed a settled transition specifically.