#93 raw policy differs between serve and _headers

What

serve and Pages disagree on /raw/* text files:

• serve: text extensions → text/plain; charset=utf-8, inline
• _headers (src/build.rs:1097): every /raw/* gets Content-Disposition: attachment, so text downloads

Impact

Same URL behaves differently across deployment shapes (the core single-shape goal). Not a security hole, but review/clone/download UX differs and the docs are ambiguous.

Fix

Pick one policy. Recommended: attachment for all raw files on both (safer, simpler), or encode the exact text-vs-active policy in _headers per extension. Update the deploy wiki and tests so both paths agree.