Update primitives

[lePereT]

What type of PR is this? (check all applicable)

• 🐛 Bug Fix
• 🧑‍💻 Code Refactor
• 🔥 Performance Improvements
• ✅ Test

Description

Tightens channel rendezvous semantics and makes waitgroups reusable; trims flakiness in tests.

• channel.lua

  • Replace ad-hoc GC of stale queue entries with explicit per-op entries and a cancellable flag.
  • Use on_abort to mark losing arms cancelled; pop_active() skips cancelled entries.
  • Preserve buffered behaviour; simplify fast paths; tidy module header.

• waitgroup.lua

  • Introduce per-generation condition; allocate on first add() from zero, signal on return to zero.
  • wait_op() rechecks after try() and blocks on the current generation only.
  • Guard against negative counters; no spurious wakeups.

• tests

  • test_channel: assert get() blocks before supplying a value.
  • test_waitgroup: add reuse test; shorten sleeps and timings to reduce runtime; adjust assertions accordingly.

Net effect: correct choice-loss cleanup on channels, waitgroups that can be reused safely, fewer retained waiters, and faster, more deterministic tests.

Manual test

• 👍 yes

Manual test description

Exercised channel put/get across buffered and unbuffered cases; verified blocked get() unblocks on subsequent put(). Verified waitgroup reuse across multiple generations; confirmed no deadlocks and expected wakeups.

Added tests?

• 👍 yes

Added to documentation?

• 🙅 no documentation needed