docs(changelog): prepare 0.8.0 release

[zikolach]

Summary

• promote the append-only output entry from Unreleased to 0.8.0 dated 2026-08-02
• document the Scala Native parser/toolchain stabilization included with the feature
• update README SBT/Mill coordinates to 0.8.0
• update all Scala CLI examples and their README to consume Maven Central 0.8.0 artifacts
• update changelog comparison links for v0.8.0

Validation

• SIGLYPH_NATIVE_CLANG=/usr/local/opt/llvm/bin/clang SIGLYPH_NATIVE_CLANGPP=/usr/local/opt/llvm/bin/clang++ mill __.compile
• mill core.test
• mill scalafmtCheck
• mill scalafixCheck
• openspec validate --all --strict — 14 passed, 0 failed
• git diff --check
• verified every tracked user-facing siglyph dependency coordinate under README/examples/docs/demos uses 0.8.0

Scala CLI published-dependency compilation is intentionally deferred until after v0.8.0 publishing because the release artifacts are not yet available on Maven Central.