feat(web-async): add portable time module

[kixelated]

Adds web_async::time so code can use timers and Instant on both native and the browser, the same way web_async::spawn already abstracts task spawning.

Why

tokio::time does not work on wasm32-unknown-unknown:

• tokio's clock is std::time::Instant::now(), which panics on wasm (no clock).
• tokio's timers need a runtime time driver, which wasm_bindgen_futures::spawn_local (what web_async::spawn uses on wasm) does not provide.

So any crate that sleeps, ticks on an interval, or reads an Instant currently can't compile-and-run on both targets. This came up making moq-net run in the browser, but it's the same problem for anything in this ecosystem.

What

time.rs is a thin cfg-switched re-export mirroring tokio::time 1:1:

• native / wasi: tokio::time::{Instant, Sleep, Interval, Timeout, MissedTickBehavior, sleep, sleep_until, interval, interval_at, timeout, timeout_at}
• browser wasm: the same names from wasmtimer, which implements that API on performance.now() + setTimeout, plus wasmtimer::std::Instant.
• Duration re-exported for convenience.

Callers swap tokio::time for web_async::time and the same code runs on both.

Notes:

• wasmtimer is under [target.'cfg(target_arch = "wasm32")'.dependencies], so native builds never pull it. Native just gains tokio's time feature.
• The cfg split (wasi grouped with native) matches spawn.rs.
• The mockable test clock (tokio::time::pause/advance) is intentionally not re-exported: it lives behind tokio's test-util feature, which shouldn't leak into normal builds, and only runs in native tests.

Test

• cargo fmt --check, cargo clippy -- -D warnings clean (native).
• Builds for both host and wasm32-unknown-unknown.

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