feat(async/unstable): move Channel to options-bag constructor and split per-operation options

[tomas-zijdemans]

• Constructor → options bag. new Channel<T>(capacity?: number) becomes new Channel<T>(options?: ChannelOptions) where ChannelOptions = { capacity?: number }. Matches the pattern already used in std, and leaves room for future construction-time options (drop policies, debug labels, etc.) without a second breaking change once stable. Per the style guide, single positional optionals are only fine when "inconceivable more will be added later". For a channel, more certainly will be.
• Split operation options. The old combined ChannelOptions (for send/receive) is split into ChannelSendOptions and ChannelReceiveOptions. Frees the ChannelOptions name for the constructor and lets each direction evolve independently.
• Class-level JSDoc summary. Documents FIFO order for both queues, the intentional close() / close(reason) asymmetry (senders always reject with ChannelClosedError so they can recover the unsent value, receivers reject with reason), undefined as a valid value, and multi-consumer fan-out.
• Pin + document multi-consumer toReadableStream(). Adds a JSDoc note that each call returns an independent consumer and values are distributed FIFO across all consumers, plus a test that asserts this.
• Minor: inline single-use #nextReceiver() helper into #deliverToReceiver.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.61%. Comparing base (f318978) to head (03146fb).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7106      +/-   ##
==========================================
+ Coverage   94.59%   94.61%   +0.01%     
==========================================
  Files         632      633       +1     
  Lines       51780    51775       -5     
  Branches     9324     9324              
==========================================
+ Hits        48981    48985       +4     
+ Misses       2225     2216       -9     
  Partials      574      574              

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[bartlomieju]

Dropping BREAKING because it's already an unstable API