066: The reading strip: how far, what this page cost, what is left

Scenario

Given an article page, which the meter charges for
When the reader scrolls
Then a hairline across the top of the viewport fills with how far they are
And a chip, bottom right, says the percentage, what this page has cost and the balance left, and opens the account
Given a page this site writes and declares free
Then there is no strip

Outcome

assets/reader/strip.css and strip.js, injected by chrome.py inside the nav block of every page with the meter (so the imported pages stay unedited for check 11), running only where <sg-meter view="page"> has a charged kind. Reads the meter's records; refreshes on its events; charges nothing. Tokens only; in the themes gate.