feat(cli): --allow-private-network for localhost development (#33) (#203)

Picks up @Timtech4u's PR #33, which has been waiting since April. The
  design (CLI flag + threading through BrowserContext + http client) is
  theirs and is correct; this is a clean rewrite on top of current main
  because repeated rebase rounds had been reverting unrelated work that
  landed in the meantime.

  Issue #4 blocked all loopback / RFC1918 / link-local fetches to prevent
  SSRF. The only escape was OBSCURA_ALLOW_PRIVATE_NETWORK=1, which is
  process-wide and clunky in pipelines. Multiple users (@Timtech4u in
  the original PR, then @duartemvix and @necrogami in the comment
  thread) reported they cannot use obscura for development against
  http://localhost:N without it.

  Adds --allow-private-network as a global CLI flag, works on fetch,
  serve, and scrape. Threads through BrowserContext::with_storage_and_network
  and the http client's validate_url. CdpContext gets a new_full
  builder and obscura_cdp::start_with_full_serve_options entry point so
  'obscura serve --allow-private-network' lets Puppeteer/Playwright
  navigate to localhost too. Default remains blocked.

  Backward compat: the env var still works (OR'd with the flag inside
  validate_url). All existing constructors and entry points unchanged.

  Verified end-to-end against a local http.server on 127.0.0.1: fetch
  and CDP Page.navigate both blocked by default, both succeed with the
  flag. Public URLs unaffected. Integration suite 28/28, full workspace
  cargo test all green.

crates/obscura-browser/src/context.rs

@@ -21,11 +21,17 @@ pub struct BrowserContext {
     /// the CDP server.
     pub allow_file_access: bool,
     pub storage_dir: Option<PathBuf>,
+    /// When true, the http client allows fetching localhost / RFC1918 /
+    /// link-local addresses. Set via `--allow-private-network` (issue #33).
+    /// Independent of `allow_file_access` because they cover different threat
+    /// models: file:// is a local file-system read, while private-network is
+    /// the broader SSRF gate from issue #4.
+    pub allow_private_network: bool,
 }
 
 impl BrowserContext {
     pub fn new(id: String) -> Self {
-        Self::_new_inner(id, None, false, None, None)
+        Self::_new_inner(id, None, false, None, None, false)
     }
 
     /// Create a BrowserContext with an optional storage directory.
@@ -35,7 +41,7 @@ impl BrowserContext {
         id: String,
         storage_dir: Option<PathBuf>,
     ) -> Self {
-        Self::_new_inner(id, None, false, None, storage_dir)
+        Self::_new_inner(id, None, false, None, storage_dir, false)
     }
 
     /// Create a BrowserContext with full options including storage_dir.
@@ -46,7 +52,21 @@ impl BrowserContext {
         user_agent: Option<String>,
         storage_dir: Option<PathBuf>,
     ) -> Self {
-        Self::_new_inner(id, proxy_url, stealth, user_agent, storage_dir)
+        Self::_new_inner(id, proxy_url, stealth, user_agent, storage_dir, false)
+    }
+
+    /// Variant that also accepts the `allow_private_network` opt-in. All
+    /// pre-existing constructors default it to `false`; callers that want the
+    /// CLI's `--allow-private-network` (issue #33) behaviour go through here.
+    pub fn with_storage_and_network(
+        id: String,
+        proxy_url: Option<String>,
+        stealth: bool,
+        user_agent: Option<String>,
+        storage_dir: Option<PathBuf>,
+        allow_private_network: bool,
+    ) -> Self {
+        Self::_new_inner(id, proxy_url, stealth, user_agent, storage_dir, allow_private_network)
     }
 
     fn _new_inner(
@@ -55,6 +75,7 @@ impl BrowserContext {
         stealth: bool,
         user_agent: Option<String>,
         storage_dir: Option<PathBuf>,
+        allow_private_network: bool,
     ) -> Self {
         let cookie_jar = Arc::new(CookieJar::new());
 
@@ -74,9 +95,10 @@ impl BrowserContext {
             }
         }
 
-        let mut client = ObscuraHttpClient::with_options(
+        let mut client = ObscuraHttpClient::with_full_options(
             cookie_jar.clone(),
             proxy_url.as_deref(),
+            allow_private_network,
         );
         if stealth {
             client.block_trackers = true;
@@ -102,6 +124,7 @@ impl BrowserContext {
             stealth,
             allow_file_access: false,
             storage_dir,
+            allow_private_network,
         }
     }
 
@@ -115,7 +138,7 @@ impl BrowserContext {
         stealth: bool,
         user_agent: Option<String>,
     ) -> Self {
-        Self::_new_inner(id, proxy_url, stealth, user_agent, None)
+        Self::_new_inner(id, proxy_url, stealth, user_agent, None, false)
     }
 
     pub fn with_proxy(id: String, proxy_url: Option<String>) -> Self {