Revise Haptics explainer based on CSSWG F2F feedback

[liminzhu]

On a high level

• Replaced pseudo-class declarative model with new model centered on @haptic-trigger, including effect/intensity/phase descriptors and enter/exit trigger semantics.
• Greatly expanded Alternatives Considered with CSS/JS/HTML options, tradeoff analysis, comparison matrix, and side-by-side appendix examples.
• Removed scroll-snap-haptics in favor of :snapped
• Added explicit conflict handling for overlapping declarative rules
• Added a shared input-device routing model and updated activation model accordingly.
• Added deferred user preference media feature discussion.
• Editorial cleanup and organization improvements