fix: preserve code-fence imports when inlining partials

[ilanazholobovsky]

Follow-up to #55.

When a partial is spliced into a page, its own component imports are removed —
@theme/Tabs and friends mean nothing in plain markdown. That strip is
line-based (/^\s*import\s+.*$/gm), so it also matches any line starting with
import inside a fenced code block:

import Foundation   // deleted from the middle of the sample

The failure is silent: the page still renders, the fence survives, and only the
import lines go missing from the generated markdown.

maskCodeSegments() already exists in this module and is used by
cleanMarkdownContent() and the image rewriter for exactly this reason.
Applying it here protects fenced blocks, tilde fences and inline spans, while
genuine component imports are still stripped since they never sit inside a fence.

Adds tests/test-partial-code-fence-imports.js (registered in test:unit),
which fails before the change — Swift and Kotlin import lines stripped — and
passes after.

[rachfop]

Landed on main in 47d10ea (rebased onto #68 so both changes to the test script survived). Apologies for the PR auto-closing — my force-push of the rebase to your fork made head and base point at the same SHA. The fix and your test file are live and credited to you. Thanks for catching the missing maskCodeSegments on the partial-splice path.