ARW: support uncompressed (type 1) and lossy (type 8) compression

[justin13888]

rawshift-image-arw decodes only lossless-JPEG ARW (compression type 7). Everything else returns RawError::Unsupported, so those files cannot be opened at all.

From crates/rawshift-image-arw/src/decoder.rs (master):

// after the `compression == 7` path
match metadata.compression {
    8 => Err(RawError::Unsupported(
        "Sony Compressed (Type 8) not yet supported. Only Uncompressed/LJPEG (Type 7) is supported."
    )),
    _ => Err(RawError::Unsupported(format!(
        "Compression type {} not yet supported (only JPEG type 7 is supported)",
        metadata.compression
    ))),
}

Gaps

• Type 1 — uncompressed. Falls through to the _ arm. 12/14-bit packed sensor data; the simplest of the three to add.
• Type 8 — Sony lossy (ARW2). 7-bit delta encoding over 32-pixel blocks with per-block min/max; well documented across dcraw, LibRaw and rawloader.

Aside

The type-8 message reads "Only Uncompressed/LJPEG (Type 7) is supported" — but uncompressed is type 1 and unsupported, so that string is misleading either way. Worth fixing with the substance.

Test coverage

rawshift-test-fixtures currently carries only lossless ILCE-6700 files, so both variants need fixtures. raw.pixls.us has CC0 samples covering uncompressed and lossy Sony bodies; happy to source and ingest them via ingest.sh if useful.

Why

Focale (Capsulsaurus/Focale#12) targets Sony ARW as its first-class format and currently surfaces a typed per-file error for these two variants. It is the only thing blocking that issue.