refactor(md): Add indent to buffer events and list streaming

[JeanMertz]

The Event enum variants have been converted from tuple structs to named-field structs, each gaining an indent: usize field that carries the visual column at which the renderer should place the content. For example, Event::Block(String) becomes Event::Block { content: String, indent: usize }.

A new InList buffer state replaces the old ad-hoc list-streaming logic inside handle_buffering_paragraph. It tracks marker_column, content_column, ordering, delimiter, and item count, and pushes/pops a parent-state stack when entering nested containers (sub-lists or fenced code blocks inside list items). Each sibling marker triggers an immediate flush of the preceding item as a Block with the list's marker_column as its visual indent, so items stream individually rather than buffering the whole list.

The Buffer::flush() -> Option<String> API is replaced by flush_events() -> Vec<Event>, which correctly splits any queued items at sibling-marker boundaries, renumbers ordered-list markers relative to start_number + items_flushed, and emits the final partial segment as a Flush event with the right indent.

TerminalOptions gains an indent field, threaded through Formatter::format_terminal_with, TerminalWriter::new, and the chat renderer's print_block / print_code / terminal_options helpers. TerminalWriter seeds its initial prefix with indent spaces so every rendered line starts at the requested visual column. A new indent_lines helper in the chat renderer prepends the indent to raw code-block lines that have already been highlighted.

The practical effect: when an LLM streams a response with nested lists or fenced code blocks inside list items, each item and code line now renders at the correct visual column as it arrives, rather than being buffered until the whole list is received or rendered at column 0.