Avoid recursive matching for object stream indexes

[NiekNijland]

Fixes #835.

The object-stream parser currently separates the object index with a nested, recursive PCRE pattern. Large valid indexes can exhaust the PCRE JIT stack; preg_match then returns false and the parser reads missing captures.

This change:

• uses the required ObjStm N and First metadata to separate the index from object bodies;
• tokenizes the bounded index with a non-recursive whitespace split;
• validates counts, numeric values, offsets, and duplicate offsets before parsing embedded objects;
• preserves existing object-stream behavior for valid PDFs.

Verification:

• regression fails before the change with Undefined array key warnings and 0 parsed objects;
• focused object-stream tests: 3 tests, 6 assertions;
• full PHPUnit suite: 195 tests, 1189 assertions;
• PHP-CS-Fixer dry-run: clean;
• PHPStan: clean.