fix: Swig tag breaks Markdown processing on the same line

[KentarouTakeda]

Note: This PR recreates #5752. I accidentally closed that PR — a force-push during a rebase auto-closed it — and GitHub no longer allows it to be reopened ("the repository may be missing relevant data"), so I'm recreating it here with the same changes. Original review thread: #5752. cc @VibhorGautam — thanks again for the suggestion, which is now included as a fix below.

What does it do?

Problem

When a Swig/Nunjucks tag appears at the beginning of a line, Markdown syntax on the rest of that line is not processed.

Input:

{% post_link my-post 'My Post' %} and [link](https://example.com)

Expected: link becomes a clickable link.
Actual: [link](https://example.com) is rendered as plain text.

Cause

Hexo replaces Swig tags with HTML comment placeholders (<!--swigN-->) before Markdown rendering. When this placeholder starts a line, it triggers an HTML block (type 2) per the CommonMark spec — the entire line is treated as raw HTML and no Markdown processing occurs (see Example 177).

Fix

A single placeholder format cannot satisfy both requirements:

• Block-level tags (e.g. {% blockquote %}) need the HTML comment placeholder to trigger HTML block parsing, which prevents the Markdown renderer from wrapping the output in <p> tags.
• Inline tags with trailing content (e.g. {% post_link %} [link](url)) must not trigger HTML block parsing, or the rest of the line loses Markdown processing.

This PR adds an inline placeholder format (swigN) that uses U+FFFC (Object Replacement Character) as delimiters instead of HTML comments. U+FFFC is already used in the existing block placeholder format and is safe for this purpose — it has no special meaning in HTML or Markdown, does not appear in normal text, and survives Markdown rendering unchanged.

The state machine in escapeAllSwigTags looks ahead after each closing %} / }}: if non-whitespace content follows on the same line, the inline format is used; otherwise the existing block format (<!--swigN-->) is used.

Additional fix: trailing Markdown after a block tag's closing

Following @VibhorGautam's review suggestion on #5752, I added a regression test for a full block tag whose closing is followed by Markdown on the same line:

{% blockquote %}quote{% endblockquote %} [link](https://example.com)

The test surfaced a real bug: the same-line Markdown was not processed, because the full-block-tag branch in escapeAllSwigTags was the only escape path that didn't pass the inline-placeholder flag — the same root cause as above, on the closing side. Fixed by applying the same hasTrailingContentOnLine look-ahead there. All post.ts tests pass (121 passing / 1 pending).

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[github-actions]

How to test

git clone -b fix/inline-swig-placeholder https://github.com/KentarouTakeda/hexojs-hexo.git
cd hexo
npm install
npm test