ODT reader: rewrite in monadic style instead of arrows.

The ODT reader was written in an idiosyncratic arrows-based style,
using a custom ArrowState arrow and dozens of special combinators.
Replace this with an ordinary monad:

    type XMLConverter nsID extraState
       = ExceptT () (State (XMLConverterState nsID extraState))

This preserves the original semantics (state changes persist through
failure; recovery via Alternative's <|>) while allowing all readers
to be written in plain do/applicative notation.

- Rewrite Generic/XMLConverter.hs around the new monad; replace the
  parent-element stack with a single currentElement field, and make
  matchContent matchers simple (namespace, name, reader) triples
  whose results are mconcat'd in document order.
- Port StyleReader.hs and ContentReader.hs to monadic style.
- constructList now runs its child reader once instead of twice.
- Delete Arrows/State.hs, Arrows/Utils.hs, and Base.hs; trim
  Generic/Fallible.hs and Generic/Utils.hs to what is still used.

Net -994 lines. No changes to test output.

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

Author: jgm

pandoc.cabal

@@ -771,7 +771,6 @@ library
                    Text.Pandoc.Readers.Mdoc.Standards,
                    Text.Pandoc.Readers.Typst.Parsing,
                    Text.Pandoc.Readers.Typst.Math,
-                   Text.Pandoc.Readers.ODT.Base,
                    Text.Pandoc.Readers.ODT.Namespaces,
                    Text.Pandoc.Readers.ODT.StyleReader,
                    Text.Pandoc.Readers.ODT.ContentReader,
@@ -780,8 +779,6 @@ library
                    Text.Pandoc.Readers.ODT.Generic.Utils,
                    Text.Pandoc.Readers.ODT.Generic.Namespaces,
                    Text.Pandoc.Readers.ODT.Generic.XMLConverter,
-                   Text.Pandoc.Readers.ODT.Arrows.State,
-                   Text.Pandoc.Readers.ODT.Arrows.Utils,
                    Text.Pandoc.Readers.Org.BlockStarts,
                    Text.Pandoc.Readers.Org.Blocks,
                    Text.Pandoc.Readers.Org.DocumentTree,

src/Text/Pandoc/Readers/ODT.hs

@@ -103,7 +103,7 @@ archiveToODT archive = do
   let media = filteredFilesFromArchive archive filePathIsODTMedia
   let startState = readerState styles media
   either (\_ -> Left $ PandocParseError "Could not convert opendocument") Right
-    (runConverter' read_body startState contentElem)
+    (runConverter read_body startState contentElem)
 
 
 --