Simplify Buffer: size is plain channel capacity

[destel]

Previously, Buffer allocated a channel of capacity size-1 to compensate for the additional item held by the forwarding goroutine. This PR removes that compensation, simplifying the code and making it consistent with the rest of the library, where no stage counts its in-flight item. Since size now means plain channel capacity, zero is a valid value, and Buffer's size validation added in #95 is removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (1c3040d) to head (e32b2ec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
- Coverage   99.22%   99.22%   -0.01%     
==========================================
  Files          13       13              
  Lines         649      648       -1     
==========================================
- Hits          644      643       -1     
  Misses          5        5              

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