fix(verify_pdf): --ascii-dates needs digit boundaries around the year (#512)

_YEAR matched (19|20)\d{2} anywhere, so `2000` inside `120000` or `12000`
made a budget or throughput range typed with `--` read as an en-dashed
date range: "Grew budget DKK 120000-200000" and "12000-15000 events/s"
both failed /apply Step 5d, and the agent was sent to fix a date argument
that does not exist. The guide promises a numeric range with no year is
left alone.

Bound the year with (?<!\d) and (?!\d). "2016-2024" next to a longer
number is still caught. Two new cases; the numeric one fails on the
unbounded pattern.

tools/verify_pdf.py

@@ -94,7 +94,9 @@ def normalize_text(text):
 # rest are the other Unicode dashes and the minus sign, which a parser that splits
 # a range only on U+002D treats the same way.
 NON_ASCII_DASHES = "\u2010\u2011\u2012\u2013\u2014\u2015\u2212"
-_YEAR = r"(?:19|20)\d{2}"
+# A year is four digits standing alone: without the digit boundaries, `2000`
+# inside `120000` or `12000` made a budget or throughput range read as a date.
+_YEAR = r"(?<!\d)(?:19|20)\d{2}(?!\d)"
 # Horizontal whitespace only between the year and the dash: `\s*` also matched
 # a newline, so a year ending one line joined a dash opening the next (a bullet,
 # a wrapped prose line) and an ASCII date was reported as U+2013.