refactor: name configurable viewport scroll steps accurately

docs/configuration.md

@@ -16,7 +16,7 @@ Vamp.configure do |config|
   config.git_blame = true # false disables current-line attribution
   config.git_gutter = "bar" # or "symbols", "line_numbers", "none"
   config.scroll_distance = 0 # nonnegative edge distance, or nil to center
-  config.scroll_half_page_lines = nil # Ctrl+u/d: half viewport, or a positive screen-row count
+  config.scroll_step_lines = nil # Ctrl+u/d: half viewport, or a positive screen-row count
   config.highlight_trailing_whitespace = false # true marks line-end whitespace in red
   config.whitespace_characters = {} # opt in: { " " => "·", "\t" => "→", "\n" => "↵" }
   config.whitespace_scope = "all" # or "leading" for indentation only
@@ -58,7 +58,7 @@ Plugin paths resolve relative to the config file, not the shell's working direct
 
 The setting applies to ordinary and historical buffers, split/unified diff panes, file pickers and explorers, command/buffer palettes, completion lists, and revision selectors. Wrapped text measures visual rows, not source lines; picker distances use result rows, excluding borders, search fields, and help. Manual mouse-wheel scrolling remains detached until keyboard navigation resumes. Only `nil` or integers from `0` through the platform's Rust `usize` maximum (`18446744073709551615` on 64-bit systems) are accepted; other values reject the configuration.
 
-`config.scroll_half_page_lines` controls Normal/Visual Ctrl+u/Ctrl+d (`core.scroll_half_up` / `core.scroll_half_down`). The default `nil` scrolls half the focused text viewport, rounded down with a minimum of one screen row. A positive integer, such as `config.scroll_half_page_lines = 3`, fixes the distance in screen rows, including wrapped continuations and diff display rows. Counts multiply that distance; `2` then Ctrl+d scrolls six rows with this example. This session-wide setting applies to ordinary, historical, diff, and conflict text panes without moving carets or selections. It does not change the cursor-edge margin, one-row scroll actions, mouse wheel, or picker navigation. Only `nil` or integers from `1` through the platform's Rust `usize` maximum are accepted; other values reject the configuration. Distances and count multiplication saturate at Rust's `isize` limit before the viewport clamps to content bounds.
+`config.scroll_step_lines` controls Normal/Visual Ctrl+u/Ctrl+d (`core.scroll_step_up` / `core.scroll_step_down`). The default `nil` scrolls half the focused text viewport, rounded down with a minimum of one screen row. A positive integer, such as `config.scroll_step_lines = 3`, fixes the distance in screen rows, including wrapped continuations and diff display rows. Counts multiply that distance; `2` then Ctrl+d scrolls six rows with this example. This session-wide setting applies to ordinary, historical, diff, and conflict text panes without moving carets or selections. It does not change the cursor-edge margin, one-row scroll actions, mouse wheel, or picker navigation. Only `nil` or integers from `1` through the platform's Rust `usize` maximum are accepted; other values reject the configuration. Distances and count multiplication saturate at Rust's `isize` limit before the viewport clamps to content bounds.
 
 `config.highlight_trailing_whitespace` defaults to `false`. Set it to `true` to give trailing whitespace a red background in ordinary and historical buffers and both sides of split/unified diffs, including wrapped and horizontally scrolled text. Spaces, expanded tab cells, and Unicode whitespace at the end of a source line are highlighted; whitespace-only lines are included, but empty lines and padding beyond the text are not. Wrapping does not make interior whitespace trailing. Search, selections, and secondary carets retain their background precedence; diagnostic underlines remain visible. This changes display only, not file contents. Only literal Ruby booleans are accepted; other values reject the configuration.
 
@@ -211,7 +211,7 @@ Normal `C` / `D` / `Y` change/delete/copy the cursor-to-line-end suffix through
 
 `f<char>` / `F<char>` seeks forward/backward on the current line in Normal and Visual, with counts such as `2fx`. Rebind `config.core.seek_forward` / `config.core.seek_backward` to a single key or leader sequence; the action then waits for a literal target character. For example, `config.bind "s", config.core.seek_forward` makes `2sx` seek the second following `x`. Pass `mode: config.modes.visual` to install a Visual route. These native-only actions do not have Ex commands.
 
-Normal and Visual Ctrl+u/Ctrl+d invoke `config.core.scroll_half_up` / `config.core.scroll_half_down` without moving carets or selection endpoints. Counts multiply the configured `scroll_half_page_lines` distance (half a viewport by default). Bind `config.core.scroll_up` / `config.core.scroll_down` for one screen row per count, for example `config.bind "ctrl+e", config.core.scroll_down` to replace clone-below in Normal. Pass `mode: config.modes.visual` to rebind Visual too. These actions work in ordinary and diff text panes, use wrapped screen rows, and have no Ex commands. Cursor navigation or editing resumes caret following.
+Normal and Visual Ctrl+u/Ctrl+d invoke `config.core.scroll_step_up` / `config.core.scroll_step_down` without moving carets or selection endpoints. Counts multiply the configured `scroll_step_lines` distance (half a viewport by default). Bind `config.core.scroll_up` / `config.core.scroll_down` for one screen row per count, for example `config.bind "ctrl+e", config.core.scroll_down` to replace clone-below in Normal. Pass `mode: config.modes.visual` to rebind Visual too. These actions work in ordinary and diff text panes, use wrapped screen rows, and have no Ex commands. Cursor navigation or editing resumes caret following.
 
 ## Action reference
 
@@ -243,7 +243,7 @@ Normal and Visual Ctrl+u/Ctrl+d invoke `config.core.scroll_half_up` / `config.co
 | Normal, Visual | `file_picker`, `file_explorer` |
 | Normal, Visual | `document_start`, `document_end`, `go_to_line` |
 | Normal, Visual | `seek_forward`, `seek_backward` |
-| Normal, Visual | `scroll_up`, `scroll_down`, `scroll_half_up`, `scroll_half_down` |
+| Normal, Visual | `scroll_up`, `scroll_down`, `scroll_step_up`, `scroll_step_down` |
 | Normal, Insert, Visual | `line_start`, `line_end` |
 | Normal, Insert, Visual, Tree | `left`, `right` |
 | Normal, Insert, Visual, Selection, Tree | `down`, `up` |