Support latest WoW clients: BLTE mode F frames and flexible MFST root header sizes

[Copilot]

Newer WoW client builds failed to extract because the library hard-rejected two things they now use: BLTE mode F blocks and root files whose MFST header isn't exactly 24 bytes.

BLTE mode F (recursive frames)

• decode_block_with_keys previously returned InvalidFormat("recursive BLTE (mode F) not supported")
• Mode F payloads are a complete nested BLTE stream; they are now decoded recursively via decode_blte_with_keys, propagating the TACT keystore so encrypted nested blocks also decrypt

Flexible MFST root header size

• detect_format required header_size == 24 to recognize the 10.1.7+ MFST layout; any build extending the header would be misclassified as the pre-10.1.7 12-byte format and misparsed
• The declared header_size field is now honored (sanity-checked to 12–1024 and within data bounds), with blocks starting at that offset

// Before: only header_size == 24 accepted
if field_at_4 == 24 && data.len() >= 24 { ... }

// After: honor the declared header size
if (12..=1024).contains(&field_at_4) && data.len() >= field_at_4 as usize {
    let header_size = field_at_4 as usize;
    ...
    Ok((format, header_size))
}

Unknown MFST versions (>2) still error with UnsupportedVersion rather than silently misparsing.

Tests and docs

• New tests: mode-F raw/zlib round-trips, invalid nested payload, extended (32-byte) MFST header, unsupported-version error
• Updated README and module docs to list mode F support