Pre-apply file diff viewer.

Renders the line-based diff between a file's current content (old) and a proposed edit (new) BEFORE the edit is applied. This is the "pre-apply confirmation over post-hoc undo" surface described in docs/proposals/in-flight/harness-spec-frontend.md: framing always reads as "this WILL change" -- a "Proposed change" header and a "Not yet applied" caption, never past tense.

Visual language

Ports the Pierre (@pierre/diffs) visual language -- docs/proposals/in-flight/pierre-diffs-analysis.md -- flattened for a terminal cell grid (one bg + one fg per cell, no alpha compositing):

• Diff paints background only; syntax tokens own foreground and are never overridden. Added rows get a dark-green bg wash, removed rows dark-red, with a brighter "emphasis" bg tier on the word-level-changed sub-ranges of a paired change row -- syntax colors show through both tiers unchanged.
• A ▌ gutter bar (green/red) replaces the classic +/- text marker, per Pierre's bars indicator style.
• Long unchanged runs (> 2 * context + 1 lines) fold into a single dim "N unchanged lines" row; context: :all disables folding.
• Split mode is borderless and label-less -- the red/green gutters carry the old/new identity -- and the unpaired side of a change block stays blank to keep the panes vertically aligned.

Line differencing is computed by Raxol.UI.Components.Harness.LineDiff (a plain LCS line diff). Intra-line word ranges come from Raxol.UI.Components.Harness.WordDiff (positional deletion/addition pairing within a changed hunk, word-level LCS, word-alt span merging). Syntax tokens come from Raxol.UI.SyntaxHighlighter (Makeup-based; language: nil skips highlighting entirely).

Props

• :path - file path shown in the header (default "").
• :old - original file content (default "").
• :new - proposed file content (default "").
• :mode - :auto (default), :unified, or :split. In :auto, split is chosen when both rendered panes fit side by side in the available width, otherwise unified. Width comes from the :width prop, else from the render context (:available_width,:width, ordimensions.width); with no width information auto falls back to unified (the safe narrow default). *:width- available width in terminal columns, used only by:auto(defaultnil). *:language- source language for syntax highlighting (e.g."elixir"), passed toRaxol.UI.SyntaxHighlighter.highlight_lines/3.nil(default) disables highlighting -- lines render as plain diff-tinted text. *:syntax_theme- Makeup style atom (e.g.:one_dark,:dracula) or a%Makeup.Styles.HTML.Style{}, passed through to the highlighter (default:one_dark). Distinct from the:themeprop below, which is the Raxol UI theme override map, not a code theme. *:context- number of unchanged lines kept visible at each edge of a folded hunk, or:allto disable folding entirely (default3). *:id,:style,:theme` - standard component props. ## Example {:ok, state} = DiffViewer.init( path: "lib/orders/total.ex", old: old_text, new: new_text, language: "elixir" ) DiffViewer.render(state, %{})