docs(editor): record source editing decision

[McanKul]

Summary

• record the Spike: evaluate safe editing of existing PDF text and images #11/spike(engine): prototype a read-only source-content capability classifier #33 compatibility matrix and conservative product boundary
• include measured classifier test time and memory with the environment and limitations stated
• compare the bounded lopdf prototype with an unimplemented out-of-process PDFium option
• link the decision from the README

Evidence

• 18 synthetic PDFs, 14,624 bytes total
• 68 classifier/preflight tests passed in 0.13 s test time
• 0.14 s wall time and 21,381,120-byte peak RSS on the documented macOS arm64 debug harness
• 325 Rust library tests passed in the merged fix(pdf): classify unsafe text states #123 revision

Closes #33
Refs #11