feat(cli): add --dump markdown output mode (#54) (#78)

Surfaces the existing LP.getMarkdown CDP machinery as a CLI flag so
text-pipeline users (LLM agents, scrapers feeding markdown to other
tools) don't need a separate HTML-to-markdown converter step.

  obscura fetch --dump markdown 'https://example.com'

Layout:
- crates/obscura-js/src/markdown.rs holds HTML_TO_MARKDOWN_JS as a
  pub const so every layer can share the same DOM-walker. obscura-js
  is at the bottom of the dep DAG (obscura-browser -> obscura-js,
  obscura-cdp -> obscura-js, obscura-cli -> obscura-js) so a single
  source of truth works without a circular dependency.
- crates/obscura-browser/src/lib.rs re-exports it for ergonomics.
- crates/obscura-cdp/src/domains/lp.rs drops its inline duplicate
  and calls the shared const.
- crates/obscura-cli/src/main.rs adds the `Markdown` DumpFormat
  variant + dump_markdown(&mut Page) which page.evaluate's the
  shared script and prints the result.

Net diff: a new 75-line markdown.rs, a 70+ line deletion in lp.rs
that's reborn as 3 lines, and ~10 lines added across the other four
files. No behavior change for `--dump html|text|links`, and
LP.getMarkdown over CDP keeps the exact same JSON shape.

Four new unit tests in obscura-js cover headings, inline emphasis +
links, ordered/unordered lists, and that <script> / <style> contents
do not leak into the output.

Generated by Ora Studio
Vibe coded by ousamabenyounes

Co-authored-by: Ora Studio <[email protected]>

crates/obscura-browser/src/lib.rs

@@ -5,3 +5,4 @@ pub mod lifecycle;
 pub use page::{Page, PageError};
 pub use context::BrowserContext;
 pub use lifecycle::{LifecycleState, WaitUntil};
+pub use obscura_js::HTML_TO_MARKDOWN_JS;