fix: serve WebP by default, order formats, keep PNG transparency

[lxsmnsyc]

Summary

Fixes the default output, which made every browser download PNG, and picks
formats per image so transparent images keep their transparency.

Changes

• The default output is now WebP and JPEG. The old default was PNG, JPEG and
  WebP, in that order. The first source was PNG, every browser reads PNG, so
  every browser downloaded the largest format and WebP was never used.
• Formats are now offered in a fixed order: AVIF, WebP, TIFF, JPEG, PNG. The
  order in output no longer matters.
• The img falls back to the last format, which is JPEG or PNG. Every browser
  reads both. The old fallback under the default was WebP.
• TIFF sorts before JPEG. Only Safari reads TIFF, so it must never be the
  fallback.
• A transparent image gets PNG in place of JPEG. JPEG has no transparency, and
  sharp painted the transparent pixels black.
• An opaque image drops PNG when JPEG is also listed, since JPEG is far smaller
  for photos. Listing PNG without JPEG keeps it.
• getImageData now reports whether an image is transparent. It checks the
  pixels only when the image has an alpha channel, because many PNGs have one
  with every pixel opaque.
• The transformer module and the fallback source both pick formats per image.

How it was tested

• pnpm test runs 164 tests across both projects and they pass. 12 are new.
• Plugin tests confirm the default output lists WebP before JPEG with no PNG,
  and that a config of PNG, JPEG, WebP and AVIF is offered as AVIF, WebP, JPEG.
• A plugin test confirms a half transparent image gets WebP and PNG, no JPEG,
  and falls back to PNG.
• A plugin test confirms an image with an alpha channel but only opaque pixels
  is treated as opaque.
• Unit tests cover the format order, the TIFF position, dropping PNG for opaque
  images, keeping PNG when it is the only broad format, and swapping JPEG for
  PNG on transparent images.
• Transformer tests confirm transparency detection for a see-through image and
  for an alpha channel with only opaque pixels.
• pnpm build succeeds.
• tsc --noEmit reports one pre-existing error in src/vite/fs.ts, unrelated to
  this change.

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commit: 626f6bc