Improve --format llm output quality on news index pages

This PR fixes three independent issues that surface when running
`webclaw --format llm` against modern news index pages. They were
all reproducible against bbc.com/news/world and reuters.com/world/middle-east
during a real briefing-generation run.

### 1. Framework hydration blobs no longer dump into the output

`to_llm_text` was unconditionally appending every parsed structured-data
item as a `## Structured Data` JSON fence. On Next.js sites, that means
the entire `__NEXT_DATA__` `pageProps` object — ad-targeting flags,
build IDs, schedule paths, feature toggles — gets serialized straight
into the LLM context. On bbc.com/news/world it was about 140 KB of
pure framework noise drowning the actual page content.

The fix layers three filters:

- Items with a Schema.org `@type` of `WebSite`, `WebPage`, or
  `SiteNavigationElement` are dropped as chrome.
- Items without an `@type` (typical of `pageProps` or SvelteKit
  data) are kept only if their serialized size stays under 4 KB —
  small parsed records with real content survive, hydration blobs
  do not.
- The whole section is suppressed if the total serialized size
  exceeds 16 KB, regardless of type. Past that threshold it is
  almost never useful to a downstream LLM.

JSON-LD records with content-bearing `@type` values (`Article`,
`NewsArticle`, `Product`, `Recipe`, `FAQPage`, `Event`, etc.) are
preserved.

### 2. Element → Text node smashing

`children_to_md` and `inline_text` only ran the `needs_separator`
check on `Element → Element` transitions. When an element rendered
text with no trailing whitespace and was followed by a sibling
text node that started with a non-whitespace character, the two
got concatenated with no separator. The same check now applies to
the `Text` branch in both functions.

### 3. Accessibility link chrome no longer leaks into prose

Sites like Reuters wrap external/new-window links with
screen-reader-only spans (e.g. `, opens new tab`, `external link`).
These have no consistent class hook, so the structural noise filter
cannot reliably catch them and they bleed into the rendered text —
sometimes dozens of times per page.

A targeted regex scrub now runs in two places: in the body cleanup
pipeline (`strip_a11y_link_chrome`, called early after `strip_leaked_js`)
and in the link-label cleaner (`clean_link_label`) so the deduplicated
`## Links` section is also clean.

### Tests

All 286 existing unit tests pass. 8 new tests cover:

- structured-data filter: chrome-type drop, oversized untyped drop,
  small untyped keep, `NewsArticle` keep
- markdown separator: `Element → Text → Element` no longer smashes
- a11y stripper: common phrasings, variant phrasings ("opens in a
  new window", "external link"), and code-fence preservation

crates/webclaw-core/src/markdown.rs

@@ -320,6 +320,9 @@ fn children_to_md(
                 }
             }
             Node::Text(text) => {
+                if !text.is_empty() && !out.is_empty() && needs_separator(&out, text) {
+                    out.push(' ');
+                }
                 out.push_str(text);
             }
             _ => {}
@@ -350,6 +353,9 @@ fn inline_text(
                 }
             }
             Node::Text(text) => {
+                if !text.is_empty() && !out.is_empty() && needs_separator(&out, text) {
+                    out.push(' ');
+                }
                 out.push_str(text);
             }
             _ => {}
@@ -1606,4 +1612,18 @@ mod tests {
             "collapse_whitespace stripped 6-space indent: {output}"
         );
     }
+
+    #[test]
+    fn text_after_inline_element_keeps_separator() {
+        // Reuters-style markup: <a><time>3h</time>ago</a><a>Tanker crosses...</a>
+        // The "ago" text node sits between two element children. Without a
+        // separator check on the Text branch, "ago" + "Tanker" would smash
+        // together as "agoTanker".
+        let html = r#"<div><span>3h</span>ago<span>Tanker crosses Strait</span></div>"#;
+        let (md, _, _) = convert_html(html, None);
+        assert!(
+            !md.contains("agoTanker"),
+            "Element->Text->Element smashed together: {md}"
+        );
+    }
 }