[matthewhadley]

The docs describe a transformer's mode as how its output is displayed, including tty "when the output contains ANSI color sequences". In 1.61 the mode is only consulted to choose which transformer applies when a mode has been requested (:preview_tty and friends). When none has, PreviewState builds the preview with preferred_mode = None, so the transformer's output goes through automatic detection: image if it decodes as one, else text.

So a transformer like this one, which renders code through bat:

{
    input_extensions: [ "lua", "toml" ]
    output_extension: txt
    mode: tty
    command: [ "sh", "-c", "bat --color=always \"$1\" > \"$2\"", "_", "{input-path}", "{output-path}" ]
}

previews as text, with bat's escape sequences shown literally ([38;2;173;92;124mlocal[0m…). :preview_tty displays it correctly, but that mode then sticks for every file: images fail with "stream did not contain valid UTF-8" and untransformed text loses its highlighting.

This change stores the transformer's mode on PreviewTransform and uses it as the fallback in the three places a transformed preview is built (PreviewState::new, set_auto_mode, and the path used when the selection moves to another file): an explicitly requested mode still wins, and files without a transformer are unaffected.

Tested with the transformer above on 1.61: .lua and .toml files preview with bat's colours, then moving to a .png shows the image and to an extensionless file shows the syntect-highlighted text, without any mode being picked by hand.

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