Resolve references to missing objects as null in the object reader

[Velli20]

Summary

ISO 32000 §7.3.10 treats a reference to a nonexistent object as null. The object reader raised MissingObject instead. Decoders worked around that one by one with is_absent checks, for page /Contents and /Annots among others. Every other optional read failed outright instead of returning None.

• Reader: ReadSession::load_object resolves an absent object to Null. ObjectReadError::MissingObject is removed. optional / optional_shared now return None for these references through the existing Option<T> null handling.
• Object collection: ObjectCollection::insert decodes stream filters while objects are still loading, where a missing object may be defined later. It now reads through a private LoadedObjects source that fails on unloaded objects, so those streams stay encoded and are decoded later, as before.
• Cleanup: the /Contents and /Annots workarounds are gone, and is_absent is now a plain null check.

Required reads of a missing object still fail, now with a TypeMismatch that reports Null. Some documents that failed to read before may now read; the conformance comment on this PR will show any changes.

Test plan

• cargo fmt --check, cargo clippy --all --workspace
• cargo test --workspace (Skia crates excluded locally); includes insert_preserves_encoded_state_when_filter_dependencies_are_unresolved
• Conformance read comparison on this PR

🤖 Generated with Claude Code

[github-actions]

Conformance reads

Documents that Safe-PDF reads differently from the base commit, compared with PDFium. A read
regresses when Safe-PDF fails to open a document it opened before, crashes or times out, or
counts a different number of pages than PDFium.

pdf.js corpus

1008 documents compared with 066c847: 1 read worse, 0 read better.

Regressions

Document	Base	This pull request
issue17190_1	read, 1 page	timeout: time limit exceeded (stage safe-read)

840 documents of the PDFium corpus read as on 066c847.

Details are in the job summaries of the workflow run. Rerun a case locally with
cargo conformance run --corpus <corpus> --read-only --case <id>.