feat(keymap): add page-sized scroll_diff actions

[umputun]

J/K scroll the diff viewport from either pane, but only by a wheel step. pgdown/pgup and ctrl+d/ctrl+u stay pane-relative, so with the tree focused they page the tree and skimming a file means tabbing into the diff and back.

Adds four page-sized siblings of scroll_diff_down/scroll_diff_up:

• scroll_diff_page_down
• scroll_diff_page_up
• scroll_diff_half_page_down
• scroll_diff_half_page_up

Dispatched in handleTreeAction, inside the early-return switch ahead of the mdTOC branch so tree and TOC selection stay put, and in handleDiffMovement so a globally mapped key does not go dead when focus moves to the diff.

No default binding, by design. pgdown/pgup/ctrl+d/ctrl+u are the only keys that page the file tree, the markdown TOC and the help/info overlays, so repointing them would take that away. Bind them yourself for lazygit-style paging:

map pgdown scroll_diff_page_down
map pgup scroll_diff_page_up

Full page uses pageRows() and honors --page-overlap. Half page uses a new halfPageRows() and does not, matching the rule the cursor-motion half-page keys already follow from #313. That helper also replaces the two inline copies of max(1, Height/2) in the existing half-page motions.

Tests drive a real keybindings file through keymap.Load and then Update: both focus modes, cursor pinned at the new viewport top, focus and tree selection unchanged, up reversing down, overlap subtracted from full page and ignored by half, TOC cursor unmoved. Docs updated in README, site/docs.html and both plugin references/config.md copies, with the remap example and its three consequences.

Most of the app/keymap/keymap.go diff is gofmt realigning the const block: the longest new identifier moved the alignment column.

Fix #357

[Copilot]

🟢 Approval recommended

The change is well-scoped, test-covered, and preserves existing defaults; only a minor comment-clarity nit was identified.

Pull request overview

Adds new keymap actions to page/half-page scroll the diff viewport even when focus is in the file tree/TOC, enabling “always scroll the diff” paging without changing any default bindings.

Changes:

• Introduces four new actions (scroll_diff_{page,half_page}_{down,up}) and registers them as valid keymap actions with help descriptions.
• Handles the new actions in both diff-focused and tree-focused action dispatch paths, reusing existing page sizing (pageRows()) and adding a shared halfPageRows() helper.
• Adds targeted tests for focus behavior, overlap handling, reverse scrolling, and TOC selection stability; updates user docs and plugin reference docs.

File summaries

File	Description
app/keymap/keymap.go	Adds new action constants, marks them valid, and provides help descriptions (still omitted from help/dump until bound).
app/keymap/keymap_test.go	Verifies validity, no default bindings, and help-section behavior once bound.
app/ui/diffnav.go	Implements page/half-page diff viewport scrolling actions in both tree and diff dispatch; adds halfPageRows() helper.
app/ui/diffnav_test.go	Adds integration-style tests that load a keybindings file and assert viewport/cursor/focus/TOC invariants.
README.md	Documents how to bind the new actions and the behavioral consequences of remapping paging keys.
site/docs.html	Mirrors README documentation updates for the website docs.
.claude-plugin/skills/revdiff/references/config.md	Updates action list and adds guidance for binding the new actions.
plugins/codex/skills/revdiff/references/config.md	Same reference-doc updates for the Codex plugin copy.

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite

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