feat: fix brace expansion quoting, add test visualization, expand tes…

[dbreunig]

…t suite

Brace expansion fixes:

• Fix partial quoting: "pre"{a,b}"suf" now correctly expands to preasuf prebsuf
• Fix escaped braces: {a,b} now correctly outputs {a,b} literally
• Add lexer segment tracking to preserve quoting boundaries across mixed quoted/unquoted word sections
• Preserve backslashes before { and } in lexer so parser creates EscapedPart nodes that prevent brace expansion
• Add parts-level brace expansion in expansion.py that operates on AST nodes instead of flattened strings, with string-level fallback for cross-part braces

Test and documentation updates:

• Add ASCII stacked bar graph test results visualization to README
• Add CLAUDE.md instruction to update test visualization on each commit
• Add comprehensive test files covering arithmetic, arrays, assignments, brace expansion, brackets, control flow, filesystem ops, globs, heredocs, pattern substitution, redirects, special vars, var ops, word splitting, and more
• Reorganize spec tests into per-tool directories (awk, bash, grep, jq, sed)

New commands and builtins:

• Add rmdir, time, and whoami commands
• Add getopts builtin

Interpreter and parser improvements:

• Expand interpreter builtins (control, declare, local, misc, unset)
• Improve conditionals, control flow, and expansion handling
• Enhance lexer and parser for better bash compatibility
• Update in-memory and overlay filesystem implementations
• Extend command implementations (awk, cat, grep, od, printf, pwd, read, readlink, sed, stat, touch)