Add engine orchestration to SX spec (fetch, triggers, swap, SSE, history, init)
29 orchestration functions written in SX + adapter style: request pipeline
(execute-request, do-fetch, handle-fetch-success), trigger binding (poll,
intersect, load, revealed, event), post-swap processing, OOB swaps, boost,
SSE, inline handlers, preload, history/popstate, and engine-init. Platform
JS implementations in bootstrap_js.py for all browser-specific operations.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>