fix(build): preserve per-version assets when deduplicating (fixes unstyled non-latest doc versions)

[maksimzinchuk]

Problem

Non-latest mike doc versions lost all styling in prod (e.g. https://docs.virtocommerce.org/platform/user-guide/2.0/). Their HTML referenced a content-hashed stylesheet (main.484c7ddc.min.css) that was absent from the image, so the server returned the SPA/index fallback (200 + content-type: text/html) and the browser ignored it as CSS.

Root cause

versioned-build-cicd.py → deduplicate_assets() replaced every nested assets/ folder with a symlink to the root assets/ (the current build, e.g. hash 6543a935).

Material uses content-hashed asset filenames. A mike version built earlier with a different Material release carries a different hash. Symlinking that version's assets/ to the current-build root assets/ hid its stylesheets, so its pages pointed at a hash that no longer existed → unstyled pages.

This is also why a future mkdocs-material bump on main would silently re-break every older version: a main deploy rebuilds only the latest version, but the dedup step collapses all versions onto the new root hash.

Fix

Only deduplicate a nested assets/ folder when all of its files are also present in the root assets/. Folders with version-specific files (an older Material build's hashed assets) are kept as real files.

• Same-build subsites still get symlinked → the ~100MB space saving is preserved.
• Cross-version (different Material) assets are preserved → no more 404s / unstyled old versions, even across Material upgrades.

Verification

Unit-tested the function on a synthetic tree (root + a same-hash 3.0 folder + an old-hash 2.0 folder):

• 3.0 assets/ → symlinked (deduplicated) ✅
• 2.0 assets/ → kept as real files; old-hash CSS still present ✅
• root assets/ intact ✅

All three live versions (1.0/2.0/3.0) currently serve content-type: text/css after the manual per-branch redeploys; this change makes that resilient to future Material upgrades without per-branch redeploys.

Refs VirtoCommerce/vc-github-actions#247

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