Line-range selection (file.ts:10-30 / --lines)

Snapshot only part of a file:
- `snipshot file.ts:10-30`
- or `--lines 10-30`

Pairs with `--line-start` so displayed numbers match the source.

- [ ] Parse `path:start-end` and `--lines`
- [ ] Slice code before render, preserve numbering

Start by finding the CLI argument parsing for `snipshot file.ts` and existing handling of `--line-start`. Add support for `path:start-end` and `--lines 10-30`, then trace where code is passed to rendering so it can be sliced before render. Done means both example commands snapshot only the requested range and displayed line numbers still match the source.

Scritto dal modello di indicizzazione a partire dal testo della issue.