Harden malformed PDF handling

[Lulzx]

Summary

• reject cyclic and excessively long XRef revision chains
• use checked parsing and conversions for attacker-controlled PDF numbers
• bound structure-tree recursion and stop reference cycles
• free empty WASM extraction buffers
• enforce ErrorConfig error budgets, logging, missing-object, and encoding policies

Why

Malformed PDFs could previously cause integer-overflow aborts, infinite XRef traversal, structure-tree stack exhaustion, or repeated WASM heap growth.

Validation

• zig build test
• zig build test -Doptimize=ReleaseSafe
• zig build wasm
• zig build -Doptimize=ReleaseSafe
• zig fmt --check build.zig gen_test.zig src/*.zig
• PYTHONPATH=python python3 -m pytest -q python/tests (211 passed, 1 skipped)