When the metadata box rebuilds its rows (on selection changes and on file updates during a save), it renders a Markdown tag tooltip for every tag row via display_tag_tooltip(). That goes through the module-level markdown.markdown() convenience function, which constructs a brand-new Markdown object, with its full processor registry, parser, and element tree, on every single call.

With many tags per file across a large bulk save/selection, this produces thousands of short-lived Markdown/Registry/ElementTree objects and pins the CPU at 100%, making Picard unresponsive (and appear to hang).
It is not a memory leak, memory stays flat, but the allocation churn is very CPU-heavy.

This is a 3.0 regression: Picard 2.x did not render Markdown tooltips per metadata-box row.

Steps to reproduce:
1. Load a large number of files/albums.
2. Select all and Save (or repeatedly change selection).
3. Observe sustained 100% CPU and UI unresponsiveness.

Regression introduced by: commit 23ef205fb ("Display tooltip on first column of metadata box items (tags)"), first shipped in 3.0.0a1. The Markdown-rendered tooltip helper came from 2355f85b0.

Proposed fix:

• Reuse a single Markdown instance (reset() + convert()) instead of the per-call convenience function.
• Cache the deterministic per-tag tooltip output (tag docs are static for the session).