fix(html): ink lands where the finger went under a fit the rects leave out

[andiwand]

Drawing on a pdf put the stroke about twice as far from the page's corner as
the finger went. OpenDocument.ios hits it on every phone; OpenDocument.droid
never did.

toBox mapped a pointer through page.getBoundingClientRect(). Every other
input to it is a rect — the glyph and run boxes the text marks are built from —
so the raw layout box is the space it is written against, and those cancel. A
pointer is the one input that does not arrive in it.

Measured in the app, on an iPhone 17 simulator:

page.getBoundingClientRect()  left 16.0   width 793.3
odr.getViewportRect(page)     left  7.8   width 386.6
page.offsetWidth              794
applied fit                   0.487

So the scale came out 0.999 where it should be 0.487, from an origin of 16.0
where the page is drawn at 7.8.

layoutScale now brings a pointer back into the rects' space before pageAt
and toBox see it. pageAt needs it too: it takes rects from pushBox and
pointers from onPointerDown, so with the spaces mixed a page far down the
list is picked wrong. It is read once per event, not per coalesced sample —
each read costs two layouts. Without viewport.js the scale is 1 and nothing
changes.

Chromium fits the page in the view itself, so odr's zoom stays 1, rectFactor()
is 1 and both rects agree. A webkit view does no such fit.

Checks

The browser suite cannot reach the case on its own, so the new check stands in
a getViewportRect that halves the box.

• with the fix: 71 checks, 0 failed
• with it reverted: 71 checks, 1 failed — the stroke lands at [45.1, 746.9]
  instead of [100, 692]

🤖 Generated with Claude Code

https://claude.ai/code/session_01XASU9ZfSJ6QwsAJGE3DrN1