Center the real-web interoperability boundary

[jt55401]

What changed

• make the historical 31.4% byte-identity result and strict v1 denominator central in the abstract, results, and conclusion
• distinguish the evaluated 123-fixture research snapshot from the current 128-fixture implementation baseline
• narrow implementation claims to the capabilities exercised by each language port
• document the exact browser lifecycle evidence boundary and seed-42 integration results
• cite the merged production content-script and Chromium walker revisions

Verification

• make check
• make paper-arxiv-docker (11-page, 304,502-byte PDF)
• no undefined citations, references, or LaTeX errors
• all 11 rendered pages inspected for clipping, overlap, and figure legibility
• arXiv bundle contents verified