RTF reader: support legacy \pn/\pntext paragraph numbering (#11686).

Word-95-style numbered and bulleted lists are encoded with a
`{\pntext ...}` auto-number destination at the start of each list
paragraph plus a `{\*\pn ...}` destination describing the numbering,
rather than the modern `\listtext`/`\listtable` mechanism.  Two problems:

1. The `\pntext` marker text ("1.", "·", etc.) was captured as the
   paragraph's first text run, which sits before the paragraph's
   `\ls`/`\ilvl`, so emitBlocks (which reads list properties from the first
   run) misclassified the paragraph as an ordinary paragraph.  The first
   item of each list therefore came out as a stray paragraph.

2. The numbering style was ignored, so numbered lists defaulted to
   bullets.

Treat `\pntext` like `\listtext`: drop its visible marker text and flag the
start of a new list item.  Parse the `{\*\pn ...}` destination
(`\pnlvlbody`/`\pnlvlblt`, `\pndec`, `\pnucltr`, `\pnlcltr`,
`\pnucrm`, `\pnlcrm`, `\pnstart`, and the `\ls`/`\ilvl` keys it
carries) into the list override table so numbered lists are
emitted as ordered lists with the right number style.

`\pn` is a paragraph property that remains in effect until reset
by `\pard`, auto-numbering every paragraph in scope. Track
this (`sPnActive`) so each paragraph becomes its own list item,
rather than merging markerless continuation paragraphs as is done
for modern `\listtext` lists.

Co-Authored-By: Claude <noreply@anthropic.com>

test/Tests/Readers/RTF.hs

@@ -36,6 +36,7 @@ tests = map rtfTest [ "footnote"
                     , "list_simple"
                     , "list_complex"
                     , "list_multiparagraph"
+                    , "list_legacy"
                     , "bookmark"
                     , "table_simple"
                     , "table_error_codes"

test/rtf/list_legacy.native

@@ -0,0 +1,14 @@
+Pandoc
+  Meta { unMeta = fromList [] }
+  [ OrderedList
+      ( 1 , Decimal , Period )
+      [ [ Para [ Str "A" ] ]
+      , [ Para [ Str "Numbered" ] ]
+      , [ Para [ Str "List" ] ]
+      ]
+  , BulletList
+      [ [ Para [ Str "A" ] ]
+      , [ Para [ Str "Bulleted" ] ]
+      , [ Para [ Str "List" ] ]
+      ]
+  ]

test/rtf/list_legacy.rtf

@@ -0,0 +1,4 @@
+{\rtf1\ansi\deff0{\fonttbl{\f0 Helvetica;}{\f2 Symbol;}{\f3 Symbol;}}
+\plain\plain\f0\fs20\pard\ssparaaux0\s0{\pntext\pard\plain\f2\fs22 1.\tab}{\*\pn\pnlvlbody\pnstart1\pndec\ls3\ilvl0\pnhang{\pntxta \'2e}}\ls3\ilvl0\fi-360\li360\ql\plain\f0\fs20 A\par Numbered\par List\par
+\pard\ssparaaux0\s0{\pntext\pard\plain\f3\fs22 \'b7\tab}{\*\pn\pnlvlblt\ls5\ilvl0\pnhang\pnf3{\pntxtb \'b7}}\ls5\ilvl0\fi-360\li360\ql\plain\f0\fs20 A\par Bulleted\par List\par
+}