README: point the verification section at veraPDF

The section previously led with a pikepdf one-liner and described
veraPDF as unavailable because it is not in Debian. It is available,
just not from apt, and it is the only check that actually tests the
PDF/UA-2 claim these templates make rather than merely confirming a
structure tree exists.

Verified against the templates: all four tagged builds pass ua2, and
the pre-fix builds of the paper and memo fail it. So this check would
have caught the untagged-but-declared-UA-2 state at any point; it was
simply never run.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README

@@ -122,16 +122,29 @@ build their headings and title blocks from the article class's own
 Verifying
 ---------
 
-To see whether a PDF really carries structure, open the tag tree in a
+Check a built PDF against the standard with veraPDF:
-PDF reader that shows one, or check for the tags directly:
+
+  verapdf -f ua2 text.pdf
+
+It prints PASS or FAIL for the file, and --format text will list the
+clauses that failed. All four of the tagged templates pass as shipped,
+so a FAIL means something in the document needs attention rather than
+something in the template.
+
+Run it. A document that declares PDF/UA-2 and does not meet it is
+worse than one that makes no claim, because the claim is what a reader
+relying on assistive software will trust. The declaration costs one
+line and validating it is the only thing that makes the line true.
+
+veraPDF is not packaged in Debian; download it from
+https://verapdf.org/ and put it on your PATH.
+
+For a quicker look at whether a PDF carries any structure at all,
+without checking conformance:
 
   python3 -c "import pikepdf,sys; d=pikepdf.open(sys.argv[1]); \
     print(d.Root.get('/StructTreeRoot') and 'tagged' or 'UNTAGGED')" file.pdf
 
-That confirms structure exists but not that the file conforms to
-PDF/UA-2. Full conformance checking needs veraPDF
-(https://verapdf.org/), which is not packaged in Debian.
-
 Posters
 -------