074: Highlights: select, tap, kept in the browser, drawn without changing the page

What is asked

"The highlighting is very important. It's very intuitive." Until now a selection could only be kept as a quote inside a pane note, in four steps, and nothing marked the page.

Scenario

Given a passage selected in an article
Then a Highlight button appears beside it
When the reader taps it
Then the passage is marked and kept in this browser, with its section
And it is still marked after a reload
When the reader taps the marked passage
Then Remove highlight takes it out

Outcome

assets/reader/highlights.js: kept under sgit.highlights.v1 as the exact words, 32 characters either side and the section (the W3C Web Annotation text quote selector), anchored by quote so it survives edits elsewhere; drawn with the CSS Custom Highlight API (::highlight(sgit-highlight)), so the imported article is unchanged; the button follows its passage as the page scrolls. Counted per section in the reading map. Not yet carried to the vault (card 076, step 1). Test: admin/build/test_reading.mjs.