Canvas · the wheel scrolls as scroll boxes do; pans hold at the press

A wheel without a modifier now slides the content by its delta, as
gpui's scroll boxes do; it moved the other way. The wheel reads the
live viewport and skips an end with no delta, and a drag pans from the
press by the pointer's whole way with the viewport held at the press,
so events that land in one frame no longer overwrite each other. Tests:
a plain wheel moves the content up with it; three wheels and an empty
end add up, and four moves pan their sum; a small frame fits at a zoom
of one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

TASKS.md

@@ -373,3 +373,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T31c | 2 | PASS | |
 | T31d | 1 | FAIL | a focused segment past the strip's fold stayed hidden; the tracker lost focus as Start and Stop swapped; Stop and Skip went untested; percent did not refuse content |
 | T31d | 2 | PASS | |
+| T32a | 1 | FAIL | the plain wheel panned against every scroll box; the handlers panned from the render's viewport, so events within a frame overwrote each other; the fit's cap at one went untested |