docs(examples): add time clock guest

[lntutor]

Summary

• add a standalone examples/time_clock guest using nx_sdk::time::now and time::monotonic
• measure a bounded, side-effect-free CPU loop guarded by black_box and log the elapsed milliseconds
• document build/run steps, clock semantics, and why a millisecond-resolution result may be 0 ms

Verification

• /Users/loi/.cargo/bin/cargo fmt --manifest-path examples/time_clock/Cargo.toml --all --check
• /Users/loi/.cargo/bin/cargo build --manifest-path examples/time_clock/Cargo.toml --release --target wasm32-unknown-unknown
• /Users/loi/.cargo/bin/cargo clippy --manifest-path examples/time_clock/Cargo.toml --target wasm32-unknown-unknown -- -D warnings
• target/debug/nx run examples/time_clock/target/wasm32-unknown-unknown/release/time_clock.wasm
• /Users/loi/.cargo/bin/cargo fmt --all --check
• /Users/loi/.cargo/bin/cargo test --workspace

Closes #68

[lntutor]

Current head 3fc371995b3d512c9aa58b50f4e53adc1b2df82b is mergeable and the visible CI surface on this exact SHA is fully green: cross-platform Check, Clippy, and Test jobs on Ubuntu, macOS, and Windows; Format; Build WASM Examples; CLI Multi-process Smoke; Benchmark Comparator Tests; Ubuntu Profiling Artifacts; Benchmark Regression Gate; and the aggregate CI Success check all completed successfully. There are no review threads or review-body comments open on the current PR state, so if this time-clock guest example still matches the intended direction, it looks ready for maintainer review/merge when convenient.