…mpgen, json, csv, tomlq, yaml, template, parallel, http, help, fc

Closes #542, #552, #553, #555, #556, #561, #564, #566, #567, #569, #570, #571, #575, #576

Adds builtin implementations for:
- rg: simplified ripgrep with recursive search
- patch: unified diff applier with -p, --dry-run, -R
- zip/unzip: archive creation/extraction with custom binary format
- iconv: character encoding conversion (utf-8, ascii, latin1, utf-16)
- compgen: bash completion generator (-W, -f, -d, -c, -v, -e)
- json: simplified jq alternative with get/set/keys/values/type
- csv: CSV utilities with headers/column/filter/count/to-json
- tomlq: TOML query tool with get/set/keys
- yaml: YAML query tool with get/set/keys
- template: mustache-lite template engine
- parallel: GNU parallel stub for virtual environment
- http: HTTPie-inspired HTTP client
- help: shell-wide help and discovery system
- fc: history listing/manipulation

All builtins include comprehensive unit tests covering positive and
negative paths.