fix(markdown): restore constructor application

[zikolach]

Summary

• restore source-compatible Markdown(...) constructor application for callers
  outside scalatui.markdown
• rename the private render-cache holder so it no longer shadows the public
  Markdown class in external source
• add an external-package regression test shared by JVM and Scala Native
• document the 0.7.0 regression under Unreleased

Root Cause

The Markdown render cache introduced a private top-level object named
Markdown. Code inside scalatui.markdown could still use the class's Scala 3
constructor application, but external callers resolved the inaccessible private
term instead, making the previously supported Markdown(...) syntax fail to
compile. Explicit new Markdown(...) remained available.

User Impact

This restores the constructor syntax used by the published Scala CLI Markdown
example and by source that compiled against 0.6.0. After merge, the fix should
be published as a 0.7.1 patch release.

Validation

• mill --no-server markdown.test markdownNative.test
• mill --no-server __.compile
• mill --no-server scalafmtCheck
• mill --no-server scalafixCheck
• openspec validate --all --strict (13 passed, 0 failed)
• git diff --check

[Copilot]

Pull request overview

This PR fixes a Scala 3 name-resolution regression in the markdown module where a private top-level term named Markdown accidentally shadowed the public Markdown class for external callers, breaking source-compatible constructor application (Markdown(...)) outside scalatui.markdown.

Changes:

• Renamed the private render-cache holder object to avoid shadowing the public Markdown class, and updated the cache entry type usages accordingly.
• Added a regression test in an external package (scalatui.markdown.consumer) to ensure Markdown(...) constructor application works for non-scalatui.markdown callers.
• Documented the regression fix under Unreleased in the changelog.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
markdown/src/scalatui/markdown/MarkdownRenderer.scala	Renames the private cache object and updates the cached entry type to restore external Markdown(...) constructor application.
markdown/test/src/scalatui/markdown/consumer/MarkdownPublicApiSuite.scala	Adds an external-package regression test that exercises Markdown(...) and validates the constructed component’s text.
CHANGELOG.md	Notes the fix under Unreleased for visibility in the next patch release.

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