all: add wait_parked, rename wait_blocked to wait_registered and bump to v0.4.0

[karalabe]

Adds TestClock::wait_parked, a barrier that waits until workers are actually parked, and renames wait_blocked to wait_registered without changing what it does.

wait_blocked counted a wait from its start until it returned. That includes the stretch after a notification or an advance has woken it but before its thread runs. A test that used it as a settling barrier could therefore move on while a woken worker was still pending, and read that worker's stale deadline. wire-rs's connection harness did exactly that, and a scheduled fuzz run caught it.

• wait_registered(n) is the old wait_blocked, under a name that says what it counts. Its behaviour stays the same, and so do next_deadline and its clamp to the current time.
• wait_parked(n) returns once at least n registered waits have their deadlines unreached or absent, and no notification on the clock is outstanding. A notification stays outstanding until every wait it ended has returned. Each signal publishes its outstanding notifications to a single counter on the clock, at constant cost per change.
• "Parked" now means only that strong state, throughout the README and the docs.

New tests hold woken workers through the crate's own hooks. They check that wait_parked waits where wait_registered does not: after a reached deadline, during an outstanding broadcast, for a pending notify_one credit, and for a notification on another condvar. One more drives a worker through the public Condvar::wait_deadline that picks a new deadline after a notification. Two new loom models cover the notified and the reached cases.

The rename breaks the API, so the version moves to 0.4.0. wire-rs, the CLI and the firmware tests need the rename. wire-rs also moves its settling barriers to wait_parked in a follow-up.