Viewer: Continuous scrolling is an independent checkbox over the base layout (#752) (#896)

* Viewer: touchpad scrolling tracks the finger 1:1 (#759)

A precision touchpad sends pixel-precise `MouseWheel` events. On Windows
they arrive with no `TouchPhase::Start`, so egui never takes its
no-smoothing "in touch" path and eases any delta of 8 px or more over
several frames, leaving the page a frame behind the finger.

The page view now applies precise (Point-unit) wheel input itself, 1:1,
through the same un-eased scroll path as keyboard and middle-button
auto-scroll, and suppresses egui's eased copy for a short window so its
internal easing tail neither doubles nor trails the motion. A coarse
mouse wheel (Line/Page units) keeps egui's smoothing, which is pleasant
for chunky notches, and single-page paging still owns the vertical axis.

Regression test: one 200 px precise swipe scrolls the full 200 px at
once (an eased path would ramp there over ~0.1 s) and leaves no drift.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

* Viewer: Continuous scrolling is an independent checkbox over the base layout (#752)

Page display matches Acrobat: the base layout (Single page or Two-page
view) is a radio choice, and Continuous scrolling is a separate checkbox,
so every combination is reachable. This also adds the previously missing
"Two-page view, one spread at a time" mode.

`PageLayout` now has just `Single` and `TwoUp`; a separate `continuous`
flag on the view (and on the default view preferences) decides whether it
scrolls. A single `display_token` ("single" | "continuous" | "two-up" |
"two-page") serialises both for settings and the control channel, keeping
the earlier `continuous`/`single`/`two-up` names working and adding
`two-page` for the new mode. The PDF's own page-layout request now maps
to the right (layout, continuous) pair, including plain TwoUp.

- View ▸ Page display and the rail menu show two radios plus a Continuous
  scrolling checkbox; Preferences gains a default Continuous checkbox.
- `view.continuous` toggles the checkbox; `view.layout.single` /
  `view.layout.two_up` switch the base layout without touching it. The
  former `view.layout.continuous` command is gone (its state is Single +
  continuous); the `layout`/`default-layout` options and a new
  `continuous` option cover scripting.
- Not scrolling, the wheel and the arrow/Page keys turn whole spreads in
  Two-page view, and only the current page or spread is laid out.

Tests: new `notches_turn_two_page_spreads`,
`two_page_view_shows_one_spread_at_a_time_until_continuous`, and updated
layout/command/persistence tests; the `view()` helper takes `continuous`.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

* Parity: the continuous-scrolling feature points at view.continuous

---------

Co-authored-by: Justin John <[email protected]>
Co-authored-by: Claude Opus 4.8 <[email protected]>
Co-authored-by: Brandon Thomas <[email protected]>

apps/pdfcraft/src/main.rs

@@ -4,10 +4,10 @@
 //! `--create-images [images…]` stages the images in one PDF and asks for the page DPI.
 //!
 //! View options (applied after the files open; also the seed of the UI control channel):
-//! `--page N  --zoom 150  --layout continuous|two-up|single  --panel comments|bookmarks|pages|fields|layers|attachments|none
+//! `--page N  --zoom 150  --layout single|continuous|two-up|two-page  --continuous on|off  --panel comments|bookmarks|pages|fields|layers|attachments|none
 //!  --theme light|dark|system  --language auto|<code>  --mode all|read|edit|convert|sign  --tool <catalogue id>  --left open|closed
 //!  --organize on  --fields on  --dialog properties|shortcuts|about  --palette <query>  --home on
-//!  --cover on|off  --default-layout continuous|two-up|single  --default-zoom fit-width|fit-page|<percent>
+//!  --cover on|off  --default-layout single|continuous|two-up|two-page  --default-continuous on|off  --default-zoom fit-width|fit-page|<percent>
 //!  --combine-view grid|list  --combine-zoom 60..200`
 //!
 //! `--new-window` opens a window of its own. Without it, on Windows, a launch that only names files