Adds a local decoder rather than the thumbhash npm package, matching the local
blurhash decoder it replaces. Pixels are pinned against evanw/thumbhash's
reference decoder via the vendored copy already in the encoder's testdata.
Unlike a blurhash, a thumbhash carries its own approximate aspect, so a record
with no dimensions now falls back to that instead of to a square.
The blurHash API field stays: Jellyfin clients and third-party native clients
still consume it.
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