fix(yamux): align default window with spec and reject invalid versions

[richard-ramos]

Summary

Align yamux’s default stream window with the spec/reference 256 KiB value (256 * 1024) instead of decimal 256000, and reject headers whose version byte is not 0.

This matches the default initial credit and version handling used by go-yamux and rust-yamux. Existing yamux tests were updated to use YamuxDefaultWindowSize for window exhaustion cases and to expect YamuxError for non-zero-version headers.

Affected Areas

• Protocol Logic
  Yamux stream-window defaults and header validation.

• Other
  Yamux muxer/header tests and metrics bucket boundaries.

Impact on Library Users

No API changes.

Behavior changes:

• fresh yamux streams now start with a 262144-byte default window;
• malformed/non-zero-version yamux headers now fail as protocol errors instead of being accepted.

Risk Assessment

Low. This brings Nim yamux behavior in line with upstream Go and Rust implementations. The main compatibility impact is that peers sending non-zero yamux versions are now rejected, which is expected for /yamux/1.0.0.

References

• Yamux specification: https://github.com/hashicorp/yamux/blob/master/spec.md
• go-yamux: https://github.com/libp2p/go-yamux
• rust-yamux: https://github.com/libp2p/rust-yamux

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.31%. Comparing base (057dfb3) to head (412c49c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2787      +/-   ##
==========================================
- Coverage   81.32%   81.31%   -0.02%     
==========================================
  Files         171      171              
  Lines       31013    31014       +1     
  Branches       12       12              
==========================================
- Hits        25221    25218       -3     
- Misses       5792     5796       +4     

Files with missing lines	Coverage Δ
libp2p/muxers/yamux/yamux.nim	91.53% <100.00%> (+0.01%)	⬆️

... and 5 files with indirect coverage changes

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