Ticking a section off crosses out the section

It struck the heading and left everything underneath looking untouched, which
is the opposite of what somebody ticking "this section is done" means. The
strike now covers every block under that heading, down to the next heading of
the same or higher rank - so a top-level section takes its sub-sections with
it.

That needed the rank, and the HTML importer was throwing it away: h2, h3 and h4
all arrived as a flat 'heading', so a parent heading had no children to cross
out. It was the visible half of a real loss - a converted article kept its
words and lost its shape. Headings now carry their level and render at it, and
a guide written before this reads as flat, exactly as it did.

Code blocks, tables and images inside a finished section fade but are not
struck. A line through a command makes it unreadable, and re-reading the
command is usually the reason to look back at a step already done.

The five imported guides were re-imported so they carry their structure.

docs/PROJECT-MAP.md

@@ -88,6 +88,6 @@ Manifest-less directories under `plugins/` are core frontend surface and always
 
 ## Size
 
-- test functions defined: **1417** (parametrised cases collect higher)
+- test functions defined: **1418** (parametrised cases collect higher)
 - documented API paths: **305** (`docs/openapi.json`, regenerate with `scripts/gen_openapi.py`)