Rename "health" references in code and HTML to neutral "activity trends" language

Start with health.py and its compute_project_health() function, then trace the related references in site/index.html and site/project.html. Rename the writer, HTML IDs and classes, JavaScript variables, and project_health.json consistently; done means the Python writer and HTML fetchers use the new activity-trends names without remaining health references.

The docs and user-visible HTML headings have been updated to use "Activity Trends" instead of "Health" — the word "health" implies a judgment call, but we're just measuring metrics and trends.

However, there are still internal references to "health" in:

Python code

• health.py filename and module
• compute_project_health() function name
• Output file: project_health.json

HTML/JS (element IDs and variable names)

• site/index.html: element IDs like health-title, health-window, health-methodology, health-method-detail, CSS classes like health-ml-header, health-git-header
• site/index.html: JS variables healthData, healthResp, healthErr, function fillHealthTable()
• site/project.html: fetches data/_cache/project_health.json

Proposed renames

• health.py → trends.py (or activity_trends.py)
• compute_project_health() → compute_activity_trends()
• project_health.json → activity_trends.json (needs coordinated update in HTML fetchers)
• HTML element IDs: health-* → trends-*
• JS variables: healthData → trendData, etc.

This is a coordinated rename — the JSON filename change means both the Python writer and the JS readers need to update together.