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[channel] Fix race condition on channel.Ready.Reset() (#446)

#### Type of change

- Bug fix

#### Description

**Root cause:** The `Ready.Reset()` method was experiencing a race
condition due to non-atomic struct field replacement. The original
implementation used `*r = *NewReady()` to reset the struct, which
performs a multi-step memory copy operation that replaces all fields
(channels `ready`, `closed`, and `once`). This struct copy is not
atomic.

Concurrently, other goroutines calling `WaitForReady()` or
`WaitForAllReady()` would read from the same channel fields (`r.ready`
and `r.closed`) via select statements. This created a classic data race
where one goroutine was writing to struct fields while others were
simultaneously reading from them.

**Race scenario:**
1. Goroutine A calls `Reset()` and begins copying new struct fields
(non-atomic multi-step operation)
2. Goroutine B calls `WaitForReady()` and attempts to read `r.ready` or
`r.closed` channels
3. Data race occurs: Goroutine A writes to memory while Goroutine B
reads from the same memory locations

**The fix:** Uses atomic pointer indirection by moving channels into an
inner `ready` struct and storing only an `atomic.Pointer[ready]` in the
outer `Ready` struct. This enables atomic swapping of the entire
internal state via `CompareAndSwap()`, eliminating the race condition
while maintaining the same external API.

#### Related issues

- discovered by #442

Signed-off-by: Liran Funaro <liran.funaro@gmail.com>

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