Add ResourceLoader to the browser crate that integrates the net, encoding,
url, and image crates into a unified resource fetching API:
- ResourceLoader wraps HttpClient for HTTP/HTTPS fetching
- Content-Type parsing determines resource type (HTML, CSS, image, other)
- WHATWG encoding sniffing for HTML (BOM > HTTP > meta prescan > default)
- BOM/HTTP charset detection for non-HTML text resources (default UTF-8)
- Relative URL resolution via base URL support
- HTTP error status handling (4xx, 5xx)
- Resource enum: Html, Css, Image, Other with appropriate metadata

30 tests covering MIME classification, text decoding (BOM, HTTP charset,
meta prescan, Windows-1252 special chars), URL resolution, and error types.

Implements issue 3mhkt6hnbhp25

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add encoding detection per WHATWG Encoding Standard and HTML spec:
- BOM sniffing for UTF-8, UTF-16LE, UTF-16BE
- HTTP Content-Type charset extraction (quoted/unquoted values)
- HTML meta prescan: scan first 1024 bytes for <meta charset> and
<meta http-equiv="Content-Type" content="...;charset=...">
- Priority: BOM > HTTP header > meta prescan > default (Windows-1252)
- UTF-16 from HTTP/meta overridden to UTF-8 per spec
- EncodingSource enum for tracking detection method confidence

39 new tests covering all detection methods and priority ordering.

Implements issue 3mhkt62ryxo2l

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Add all 27 WHATWG single-byte encodings to the encoding crate:
- Windows-1252 (with ISO-8859-1 and US-ASCII aliases per WHATWG spec)
- ISO-8859-2 through ISO-8859-16 (excluding 8859-9 which maps to Windows-1254)
- Windows-874, Windows-1250 through Windows-1258
- KOI8-R, KOI8-U
- macintosh, x-mac-cyrillic
- IBM866

Each encoding uses a 128-entry u16 lookup table for bytes 0x80-0xFF.
All WHATWG label aliases are registered for case-insensitive lookup.
32 new tests covering per-encoding correctness and error handling.

No external dependencies, no unsafe.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>