[codex] export wasm time helpers

[kixelated]

Summary

• Add web_async::time as a cross-target time facade.
• Export SystemTime/UNIX_EPOCH via std::time on native/WASI and wasmtimer::std on browser WASM.
• Add MaybeSend, MaybeSync, MaybeSendBoxFuture, and MaybeFutureExt helpers so downstream crates can avoid their own Send/Sync cfg switches.
• Update spawn to use the shared MaybeSend abstraction.
• Force reinstall CI cargo subcommands in just setup-tools so stale tool-cache metadata cannot leave cargo shear missing.

Why

The MoQ browser WASM work needs a stable way to anchor timestamps without falling back to local cfg-specific clock code, and it also needs reusable Send-local helper traits for browser-only transport futures.

The previous CI run failed after check/clippy/fmt passed because cargo binstall reported cargo-shear as already installed while the cargo shear subcommand was absent. Forcing tool installation makes the setup step repair that cache state.

Validation

• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all --check
• cargo shear
• cargo sort --workspace --check
• cargo check -p web-async --target aarch64-apple-darwin
• cargo check -p web-async --target wasm32-unknown-unknown
• cargo check -p web-async --features tracing --target aarch64-apple-darwin
• cargo check -p web-async --features tracing --target wasm32-unknown-unknown