fix(pdf): read a literal string with nested parentheses

[andiwand]

🤖 Generated with Claude Code

A public pdf printed from Firefox (https://opendocument.app/, cairo 1.18.4)
failed to render at all — map::at: key not found, thrown where
parse_document_impl reaches for a /Root that recovery never found.

The cause is one line of ISO 32000-1 7.3.4.2: balanced pairs of parentheses
within a string require no special treatment. read_string ended the string at
the first ), so (cairo 1.18.4 (https://cairographics.org)) stopped mid-value
and the rest of the dictionary parsed as garbage. That dictionary is the /Info
of an object stream whose members include the document catalog — so the parse of
the catalog's own object stream threw, the xref recovery scan hit the same wall
looking for /Type /Catalog, and the retry ran into a trailer with no /Root.

read_string now tracks nesting depth and returns only on the ) closing the
outermost pair. An escaped parenthesis goes through the escape branch and never
touches the depth.

Also here: ViewStream gets the seeking its comment said it did not have.
FileParser::read_object_stream seeks to each member's absolute position, and
std::streambuf's default seekoff fails, so the seek was silently a no-op and
the members were read in order instead. That happens to be right for every file
we have, and wrong for any object stream whose members are not laid out as its
header lists them — nothing in the spec requires them to be.

What changes for a reader

Beyond the new file rendering, three pdfs in the private set get their document
metadata back. It was all-or-nothing (pdf/AGENTS.md), and a nested-parenthesis
producer string had been throwing the whole thing away:

• basic_text.pdf, test_fail.pdf — pdfTeX-1.40.14, whose /PTEX.Fullbanner
  nests parentheses
• text_visibility.pdf — cairo 1.13.1 (http://cairographics.org)

Tests

• PdfObjectParser.standard_string_balanced_parentheses and ViewStream.seek
  / ViewStream.seek_out_of_range — new unit tests
• pdf/opendocument-app-website.pdf added to the public input set, with its
  reference output; the three private meta.json files regenerated
• Full suite green (942 passed, 8 skipped)