fix: make ctrl+d/u scroll half-page instead of full page

[rashpile]

Summary

Splits ctrl+d/ctrl+u from PgDn/PgUp so they scroll half a page, matching vim/less/tmux convention.

Closes #24

What's changed

• ctrl+d/ctrl+u now scrolls half the viewport height in all three panes (diff, tree, TOC)
• PgDn/PgUp unchanged — still scrolls full page
• Help overlay text was already correct ("half-page down / up")

Test plan

• Unit tests for ctrl+d/u moving half page in diff pane
• Unit tests for ctrl+d/u moving half page in tree pane
• Verify PgDn/PgUp still moves full page
• Full test suite passes

[Copilot]

Pull request overview

Updates key navigation so Ctrl+d / Ctrl+u perform half-page movement while keeping PgDn / PgUp as full-page paging, aligning behavior with common terminal navigators (vim/less/tmux).

Changes:

• Split Ctrl+d/u handling from PgDn/PgUp in tree, TOC, and diff navigation.
• Added half-page diff cursor movement helpers and expanded unit tests for new behavior.
• Updated user-facing docs to describe half-page scrolling for Ctrl+d/u.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
ui/model.go	Separates Ctrl+d/u from PgDn/PgUp across panes; routes to half-page movement.
ui/diffview.go	Introduces half-page diff cursor movement functions.
ui/model_test.go	Updates existing paging tests and adds new tests for half-page movement.
README.md	Documents Ctrl+d/u as half-page scroll.
.claude-plugin/skills/revdiff/references/usage.md	Mirrors README keybinding doc update.

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[umputun]

nice and clean change, thx. one real issue though.

moveDiffCursorHalfPageDown/Up use syncViewportToCursor() which only adjusts viewport when cursor leaves the visible area. with a typical viewport of ~36 lines, first ctrl+d moves cursor to line 18 but viewport stays at offset 0 since line 18 is still visible. in vim/less ctrl+d scrolls both viewport and cursor by half a page simultaneously.

the fix should be straightforward - use explicit YOffset adjustment like the existing full-page functions do, but with halfPage offset instead of viewport.Height. smth like:

// half-page down
m.viewport.SetYOffset(min(m.viewport.YOffset+halfPage, maxOffset))

// half-page up
m.viewport.SetYOffset(max(0, m.viewport.YOffset-halfPage))

also worth adding viewport.YOffset assertions to the half-page tests - currently they only check cursor position, which is why the viewport issue wasn't caught.

btw, Copilot flagged the same thing in inline comments 1, 2 and 5 - those are valid. the other two (about max(1, ...) in tests) are noise since test viewport is always large enough.

[rashpile]

Rebased on latest master.

Fixed the viewport scrolling issue — good catch. syncViewportToCursor() only adjusts when cursor leaves the visible area, so the first ctrl+d would move the cursor but the viewport stayed put.

Now both moveDiffCursorHalfPageDown/Up use explicit SetYOffset adjustment (like the full-page functions do), so cursor and viewport scroll simultaneously by half a page.

Added viewport.YOffset assertions to both ctrl+d and ctrl+u tests to prevent regressions.

[umputun]

lgtm, viewport scrolling fix looks correct. thx