Transient Caching. Minor Privacy Enhancement

[greg-randall]

Context

I added transient caching for markdown output and hid the alternate link tag for password-protected posts.

• The markdown rendering pipeline (HTML conversion, frontmatter generation) runs on every request. Caching skips that work when the post hasn't changed.
• Password-protected posts were exposing a <link rel="alternate"> tag in the HTML head, which reveals that a markdown endpoint exists for gated content.

Summary

This PR can be summarized in the following changelog entry:

• Cache rendered markdown output using transients (24h default), invalidated when the post is updated.
• Hide <link rel="alternate"> tag in the HTML head for password-protected posts.
• Add markdown_alternate_cache_expiration filter for adjusting cache duration.

Relevant technical choices:

• Cache validation checks post_modified rather than relying only on time-based expiry, so post edits show up without waiting for the cache to expire.
• Uses the transients API rather than the object cache directly. This works on any host, even without a persistent object cache.
• Uses post_password_required() for the visibility check because it respects the cookie-based password flow- it returns false if the user has already entered the password.

Test instructions

1. Visit any post or page at its .md URL and reload; second request should be served from cache.
2. Edit the post, save, and reload the .md URL, content should update.
3. Set a password on a post and verify the <link rel="alternate"> tag is gone from the HTML source. Remove the password and confirm it reappears.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Block/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities.
• I have added unit tests to verify the code works as intended.
• I have checked that the base branch is correctly set.

Fixes #

[jdevalk]

Consolidation note: overlaps with #11 in ContentRenderer. Recommend choosing a primary renderer PR and rebasing the other after merge.