feat: rebuild diff review on Pierre CodeView

[chuks-qua]

What

Previously the review surface rendered diffs through hand-rolled unified and side-by-side components, the Files pane floated over content at narrow widths, line comments used a bespoke editor inside diff rows, and the turn summary listed flat full paths.

Now the whole comparison renders through @pierre/diffs CodeView with the library's own virtualization, the Files pane is docked with minimum widths and drag-to-collapse, line comments compose inline at the clicked line using the same editor primitives as the "Add comment" flow, and the turn summary groups files under a collapsible folder tree with resolved per-file stats.

Verified: focused Vitest suites pass (turn summary 8/8, diff panel and file list suites), typecheck and lint clean, and the flows below were exercised live in the app (screenshots and video attached).

Known limits: the pull-request diff surface is unchanged in this pass, and vlist still owns transcript virtualization (Pierre owns diff virtualization).

Why

The old diff view had accumulated usability and maintenance problems: duplicated custom renderers, a floating panel that broke at narrow widths, a one-off comment editor, and summary rows that repeated full paths.

UI Changes

Before: hand-rolled diff rows, a flat summary, and a separate comment card.

After: Pierre-rendered diff, folder-tree summary, and the comment editor inline at the line.

Video: gutter + mounts the editor at the line, typing survives scroll virtualization, and saving produces the comment chip.

diff-comment-flow.webm

Key Changes

• Pierre CodeView for review diffs: ReviewDiffView renders the whole comparison through CodeView, replacing UnifiedDiff, SideBySideDiff, FileEntry, DiffContent, and useDiffHighlighter. @pierre/diffs pinned at 1.4.1.
• Docked Files pane: no floating overlay. Both panes keep minimum widths; dragging past the minimum collapses the pane; it hides below the combined minimum and returns when there is room.
• Merged toolbar: file controls (jump, unified/split, refresh, options) portal into the top toolbar row instead of a second row.
• Inline diff comments: the gutter + sets a store-backed edit target and the editor renders inside the diff row at that line, sharing the CommentEditorComposer primitives with "Add comment". A module-level draft cache keeps unsent text when pierre's virtualizer unmounts the row.
• Folder-tree turn summary: collapsible directory headers with compressed single-child chains, indent guides, per-file stats with an em dash for zero sides, aggregate totals in the header, and a 5-file cap with a +N more files escape.
• Windows paths: path splitting and basenames handle \ separators so snapshot file lists on Windows group and resolve stats correctly.
• Plumbing: snapshot.getDiffStats and cumulative stats through contracts and transport; DiffStats gains changeType.

Config Changes

None

Review Notes

• The throwaway HTML prototype used for the redesign review is intentionally not committed.
• The dev runtime repeatedly shut down during verification; final evidence was captured against a fresh agent:up session.
• No benchmark run; Pierre's virtualization is the perf mechanism by design.