Add opt-in reactive request activation

[fc-anjos]

Summary

• add an opt-in request activation policy and upkeep_reactive only: controller macro
• keep write capture global so mutations from unmarked actions still update subscribed pages
• limit live subscription capture to HTML GET and HEAD responses
• preserve fragment targeting for partials with silent ERB preambles and choose dom_id locals independent of local order
• generate relative JavaScript imports for non-importmap applications
• release as 0.2.1

Verification

• bundle exec rake
• Pulse Milestone request/controller slice: 75 examples, 0 failures
• two-tab browser test with two successive Milestone edits propagated to both tabs