Refactor KEX handling to async and offload X25519 crypto

[vicajilau]

Summary

Related issue #23

• make SSH transport packet handling asynchronous while preserving message order
• convert KEX message handlers to async
• offload X25519 keypair creation and shared-secret computation to isolate-backed compute helpers
• keep non-isolate fallback implementation for unsupported platforms

Why

Addresses UI/main-isolate blocking reported in issue #23 by moving the heaviest X25519 operations off the main isolate and allowing async handling in the transport path.

Testing

• dart test test/src/kex test/src/ssh_transport_version_test.dart test/src/ssh_client_test.dart test/src/ssh_client_run_with_result_test.dart test/src/ssh_transport_aead_test.dart

[codecov]

Codecov Report

❌ Patch coverage is 77.50000% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.52%. Comparing base (c094d40) to head (d1306be).
⚠️ Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/ssh_transport.dart	42.85%	24 Missing ⚠️
lib/src/kex/kex_nist.dart	92.85%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   53.90%   54.52%   +0.61%     
==========================================
  Files          64       66       +2     
  Lines        5272     5357      +85     
==========================================
+ Hits         2842     2921      +79     
- Misses       2430     2436       +6     

Flag	Coverage Δ
unittests	54.52% <77.50%> (+0.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/src/kex/kex_dh.dart	100.00% <100.00%> (ø)
lib/src/kex/kex_x25519.dart	93.75% <100.00%> (+4.86%)	⬆️
lib/src/ssh_key_pair.dart	79.32% <ø> (ø)
lib/src/utils/compute.dart	100.00% <100.00%> (ø)
lib/src/utils/compute_io.dart	100.00% <100.00%> (ø)
lib/src/kex/kex_nist.dart	94.54% <92.85%> (-5.46%)	⬇️
lib/src/ssh_transport.dart	50.92% <42.85%> (+0.32%)	⬆️

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[vicajilau]

I changed SSH transport packet processing to be asynchronous, and I moved X25519 key exchange off the main isolate to avoid UI blocking. This changes the timing of handshake and packet handling; other key exchange algorithms still run synchronously.