test(runtime): assert guest-observed wasi:clocks time under a ManualClock override

[mfw78]

What

Add a small clock-reader guest fixture under modules/fixtures/ that on every event reads std::time::SystemTime::now() (which routes to wasi:clocks/wall-clock under wasm32-wasip2) and logs the wall time as whole seconds. Add a nexum-runtime harness test, harness_guest_observes_the_clock_override, that pins a ManualClock to a known instant, boots the fixture under that override, dispatches a block, and asserts the guest logged the pinned instant.

The fixture is registered in the workspace members, the just ci wasm build list, and the CI test job's wasm build list so the integration test always has its artefact.

Why

e2e_manual_clock_override_boots_and_dispatches only proves the WasiClockOverride threads through boot on the host side; nothing asserted that a guest reading wasi:clocks observes the overridden time. This closes that gap with an end-to-end assertion: the logged value equals the pinned instant and not the ambient host clock, so the override is proven to reach the guest.

This is authored as a top-of-train car so the new workspace member and fixture do not conflict-cascade against the in-flight epic stack.

Closes #281

Testing

• Built the fixture: cargo build -p clock-reader --target wasm32-wasip2 --release. Confirmed the emitted component imports wasi:clocks/wall-clock@0.2.6 via wasm-tools component wit.
• cargo test -p nexum-runtime --all-features harness_guest_observes_the_clock_override passes: the guest logs clock wall 1700000000, parsed back and asserted equal to the pinned seconds.
• Falsified the assertion by removing the clock().set(...) call: the override then defaults to the epoch, the guest logs 0, and the test fails on the timeout, confirming the assertion is live and the guest is bound to the override rather than the ambient clock.
• cargo clippy -p nexum-runtime --all-targets --features test-utils -- -D warnings and cargo clippy -p clock-reader --all-targets -- -D warnings are clean.
• cargo fmt -p nexum-runtime -p clock-reader -- --check reports only a pre-existing, unrelated drift in builder.rs (untouched here).

AI Assistance

AI Assistance: Claude Code (Opus 4.8) used for the fixture, test and PR authoring.

[jean-neiverth]

Clean test. The ManualClock sharing via Arc is correct, the timeout prevents hangs, and the CI wiring is consistent.