Bundled Header's `resolveAsset('img/logo.png')` and useResponsiveImage's
`${basePath}.${format}` fallback both expect the original at the bare
flat path. After beta.22's pipeline migration moved originals into
_source/, every bare-path lookup 404'd. The pipeline now writes a
byte-identical copy of each source to the flat dir alongside its
variant matrix, restoring backward compatibility without per-site
workarounds. mtime-skipping applies to the copy.

Bumps to 1.0.0-beta.26.

Tests: 558 passing (1 added for copy-byte-identicality).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>