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 65.33333% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.99%. Comparing base (f9fa680) to head (3c07467).

Files with missing lines	Patch %	Lines
lib/src/ssh_transport.dart	56.25%	14 Missing ⚠️
lib/src/kex/kex_x25519.dart	70.00%	6 Missing ⚠️
lib/src/utils/compute_io.dart	71.42%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   52.80%   52.99%   +0.19%     
==========================================
  Files          63       65       +2     
  Lines        5121     5168      +47     
==========================================
+ Hits         2704     2739      +35     
- Misses       2417     2429      +12     

Flag	Coverage Δ
unittests	52.99% <65.33%> (+0.19%)	⬆️

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

Files with missing lines	Coverage Δ
lib/src/utils/compute.dart	100.00% <100.00%> (ø)
lib/src/kex/kex_x25519.dart	77.14% <70.00%> (-11.75%)	⬇️
lib/src/utils/compute_io.dart	71.42% <71.42%> (ø)
lib/src/ssh_transport.dart	51.45% <56.25%> (+0.58%)	⬆️

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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.