chore(release): prepare 0.6.8

[leehack]

Summary

• bump the package and install docs from 0.6.7 to 0.6.8 so the release branch matches the next publish target
• document the leehack/llamadart-native@b8480 sync in the changelog and recent release notes
• update pinned native tag references in the README and support matrix so release docs match the shipped hook configuration

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test
• ./tool/docs/build_site.sh
• ./tool/docs/validate_links.sh

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.90%. Comparing base (c2714a2) to head (876ec6f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #94   +/-   ##
=======================================
  Coverage   76.90%   76.90%           
=======================================
  Files          67       67           
  Lines        8368     8368           
=======================================
  Hits         6435     6435           
  Misses       1933     1933           

Flag	Coverage Δ
unittests	76.90% <ø> (ø)

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