fix(net,browser): detect response charset, decode non-UTF-8 pages (#182)

obscura was calling String::from_utf8_lossy on every response body, which
silently corrupts every non-UTF-8 page (GBK, Big5, Shift-JIS, Windows-125x,
EUC-KR, ISO-8859-x). Whole language families came through as replacement
characters.

Add an encoding module (obscura-net::encoding) that picks the decoder
the way real browsers do per HTML5 § 8.2.2.4:

  1. HTTP Content-Type charset=... parameter (authoritative when present)
  2. <meta charset> or <meta http-equiv=Content-Type> sniffed from the
     first 1KB of the body (HTML only)
  3. UTF-8 fallback

Use it in:
  - obscura-net Response::text (signature changed: String, not Result)
  - obscura-browser page.rs HTML body decode (the primary scrape path)
  - obscura-browser page.rs JS body decode
  - obscura-browser page.rs CSS body decode

For JS / CSS we deliberately skip the meta sniff: those bodies are not
HTML and any '<meta charset=...>' substring inside them is not a charset
declaration.

Adds 7 unit tests for the encoding module covering header precedence,
quoted charset values, meta charset sniff, http-equiv form, the 1KB
sniff window, and a round-trip decode of GBK bytes. Adds a Python
feature test exercising file:// + meta charset and a live HTTP server
serving Shift_JIS with the wrong meta tag.

Closes #113.

crates/obscura-net/src/lib.rs

@@ -1,5 +1,6 @@
 pub mod client;
 pub mod cookies;
+pub mod encoding;
 pub mod interceptor;
 pub mod robots;
 pub mod blocklist;
@@ -8,6 +9,7 @@ pub mod wreq_client;
 
 pub use client::{ObscuraHttpClient, ObscuraNetError, RequestInfo, ResourceType, Response};
 pub use cookies::{CookieInfo, CookieJar};
+pub use encoding::{decode_non_html, decode_response};
 pub use robots::RobotsCache;
 pub use blocklist::is_blocked as is_tracker_blocked;
 #[cfg(feature = "stealth")]