Convert a Classic Editor post to blocks without losing formatting

I'm cleaning up an old post that was written in the Classic Editor back in 2019.
Its raw content (from the database, `wp_posts.post_content`) looks like this:

[paste the raw post_content here, including any [gallery ids="12,14,19"] or
[caption id="attachment_45" align="alignleft" width="300"]<img ...>Caption
text[/caption] shortcodes, and any <img style="float:left; margin:10px"> tags]

Convert this into native block markup, not a single Classic block wrapping the
whole thing (that "works" but defeats the point of migrating). Work through it
carefully:

1. Find every `[caption]` shortcode and convert it to a `wp:image` block with the
   caption text moved into the block's built-in caption field, keeping the
   align attribute (alignleft/alignright/aligncenter) as the block's align
   setting rather than leaving it as inline CSS.
2. Find every `[gallery ids="..."]` shortcode and convert it to a `wp:gallery`
   block referencing those same attachment IDs, preserving the original order.
   If an ID in the list no longer resolves to a real attachment, don't silently
   drop it, flag it instead since the image may just need re-uploading.
3. Convert `<img style="float:left">`-style inline alignment into block align
   settings, and strip the inline style attribute entirely once converted, leftover
   inline styles are the most common reason a "converted" post still looks broken.
4. Turn plain paragraphs, headings (even ones that were just `<strong>` wrapped in
   a paragraph in the old editor), and blockquotes into their matching blocks.
5. Any shortcode you don't recognize (a custom one from a retired plugin, for
   example), leave it untouched and flag it rather than guessing at a block
   equivalent, since guessing wrong can silently drop functionality.
6. Output full block comment markup ready for the code editor view, and list
   out anything you flagged in step 2 or step 5 separately below the markup.