v6.44.0

v6.44.0 - Complete Parser Bug Fixes

All 23 parser bugs discovered via probar TUI testing have been fixed.

- Nested parameter expansion ${foo:-${bar:-default}}
- Full arithmetic operators (ternary, bitwise, comma)
- Case fall-through (;&) and resume (;;&)
- Function with subshell body myfunc() ( cmd )
- Coproc syntax (new keyword)
- Extended globs @(), !(), []
- Heredoc improvements (quoted, indented)

- 7365 tests passing (zero regressions)
- Clippy clean
- Book documentation updated

See CHANGELOG.md for full details.