fix(cdp): gate Target.createTarget file:// like Page.navigate

GHSA-q55h-vfv9-qcr5 patched Page.navigate to refuse file:// URLs
unless --allow-file-access was set, but Target.createTarget accepted
the same scheme and called page.navigate(url) directly, bypassing
the policy. A CDP client could call Target.createTarget {url: file://...}
then Runtime.evaluate to read document.body.textContent and recover
arbitrary local files.

Apply the same gate at the Target.createTarget entrypoint. Pull
the file-scheme detector into a shared util module so both domains
stay in lock-step instead of one drifting from the other.

Verified blocked vs the unpatched build (pr69):
  - Patched: Target.createTarget {url: file:///tmp/secret} errors
    "Target.createTarget to file:// is disabled. ..." before any
    navigation runs.
  - Unpatched: same call returns the file contents via
    Runtime.evaluate {expression: "document.body.textContent"}.

crates/obscura-cdp/src/util.rs

@@ -0,0 +1,62 @@
+/// Helpers shared across CDP domain handlers.
+///
+/// The file-scheme detector here is reused by every CDP entrypoint that can
+/// trigger a navigation, so we don't end up with one domain enforcing the
+/// `--allow-file-access` gate and another silently letting `file://` through
+/// (see GHSA-q55h-vfv9-qcr5 and its incomplete-fix variant in
+/// `Target.createTarget`).
+
+/// Returns true when `raw` parses as a `file:`-scheme URL, or syntactically
+/// starts with `file:` after a possible leading-whitespace strip. Matching is
+/// case-insensitive on the scheme so neither `FILE://` nor `File://` slips
+/// past callers that gate on `file://`.
+pub(crate) fn url_is_file_scheme(raw: &str) -> bool {
+    url::Url::parse(raw)
+        .map(|u| u.scheme().eq_ignore_ascii_case("file"))
+        .unwrap_or_else(|_| {
+            raw.trim_start().to_ascii_lowercase().starts_with("file:")
+        })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn matches_plain_file_url() {
+        assert!(url_is_file_scheme("file:///etc/passwd"));
+    }
+
+    #[test]
+    fn matches_case_insensitively() {
+        assert!(url_is_file_scheme("FILE:///etc/passwd"));
+        assert!(url_is_file_scheme("File:///etc/passwd"));
+        assert!(url_is_file_scheme("fIlE:///etc/passwd"));
+    }
+
+    #[test]
+    fn matches_with_leading_whitespace_fallback() {
+        // url::Url::parse rejects leading whitespace, but the syntactic
+        // fallback still catches `   file:...` so callers can't be tricked
+        // into letting it through.
+        assert!(url_is_file_scheme("   file:///etc/passwd"));
+    }
+
+    #[test]
+    fn rejects_http_https_about_data() {
+        assert!(!url_is_file_scheme("http://example.com"));
+        assert!(!url_is_file_scheme("https://example.com"));
+        assert!(!url_is_file_scheme("about:blank"));
+        assert!(!url_is_file_scheme("data:text/plain,hi"));
+        assert!(!url_is_file_scheme(""));
+    }
+
+    #[test]
+    fn rejects_lookalikes_that_are_not_file_scheme() {
+        // The URL parser rejects these (no `://`), so the syntactic fallback
+        // kicks in. `file` appearing anywhere except as the leading scheme
+        // must not match.
+        assert!(!url_is_file_scheme("notfile:///x"));
+        assert!(!url_is_file_scheme("http://file/"));
+    }
+}