feat(web): render write/edit tool output as a diff card

The write/edit tools already declare card:'diff' with applied hunks on
callView/resultView, but the Web client discarded it: a mutation landed on
GenericToolCard and the details panel flattened the result to a <pre>. Add
DiffBlock (ui-primitives), diff-card-model (the single callView/resultView
derivation), and FileMutationRow (keyed under write and edit), and make the
generic fallback row and the details panel diff-aware. The +/- block form,
per-file path header, same-file gap, and footer mirror the TUI diff card;
the chat row caps at CHAT_DIFF_MAX_LINES against the panel's full height.

packages/client/ui-primitives/README.md

@@ -2,7 +2,7 @@
 
 English | [中文](README.zh.md)
 
-Pure React atoms (zero cordis): StateDot, ic_ds_* icons, Button/Pill/Menu/Modal/Input, the markdown family (MessageText/MarkdownText/JsonBlock), the read-only JsonTree inspector, the `useAnchoredMaxHeight` hook that clamps a bottom-anchored overlay to the viewport space above its anchor (re-measured on resize, scroll, and a caller-supplied dependency), and TerminalBlock. Contract: api-contracts v3 §8.
+Pure React atoms (zero cordis): StateDot, ic_ds_* icons, Button/Pill/Menu/Modal/Input, the markdown family (MessageText/MarkdownText/JsonBlock), the read-only JsonTree inspector, the `useAnchoredMaxHeight` hook that clamps a bottom-anchored overlay to the viewport space above its anchor (re-measured on resize, scroll, and a caller-supplied dependency), TerminalBlock, and DiffBlock. Contract: api-contracts v3 §8.
 
 ## Markdown rendering
 
@@ -12,6 +12,10 @@ Pure React atoms (zero cordis): StateDot, ic_ds_* icons, Button/Pill/Menu/Modal/
 
 `TerminalBlock` renders a shell command as a terminal surface: one prompt row per line of the command (the shortened `cwd` label on the first row only, since the view knows one working directory and a `cd` moves later lines elsewhere, then that line), the command's output, a status pill for a non-zero exit code or a terminating signal, and a copy control that writes the raw `output` prop. A run-state `StateDot` marks the call once, on the first row, out of flow in a gutter the card reserves as its own left padding, so the dot sits inside the card box yet left of the prompt text. It reaches three of `StateDot`'s states — the chase while `running`, red for the same exit status that renders the pill, green otherwise — so a card states whether its command is still running rather than leaving that to be inferred from the presence of output; it carries one visually hidden text label because `StateDot` is `aria-hidden`. One dot regardless of line count is deliberate: the exit status is the whole call's, so a dot per line would claim a per-line outcome the view does not carry. Command text is `white-space: pre`, so repeated spaces, tabs, and an indented continuation render verbatim while the row stays single-line and ellipsizes. ANSI escape sequences are parsed with the `anser` runtime dependency into React spans; cursor movements replay into a per-line column buffer before inert controls are stripped, since carriage return and backspace only MOVE the cursor: `100%` + CR + `OK` alone shows `OK0%`, while the `\x1b[K` a spinner writes with its redraw erases the tail so `100%\r\x1b[KOK` shows `OK`. Erase-in-line is honored in all three parameter forms, the cursor advances by terminal columns (8-column tab stops, two for emoji and CJK, none for a combining mark), and SGR state is normalized per cell as a terminal stores it, threading across lines and closing at the state the line ended in; basic-16 foreground colors map onto `--dsw-*` tokens, while 256-palette and truecolor values pass through as literal rgb. Output keeps `white-space: pre` with horizontal scrolling, so column-aligned output holds its alignment instead of soft-wrapping, and collapses to a head slice plus a tail slice past `maxLines` (default 16, the TUI transcript's split arithmetic) behind an expand button. Rationale: [the web terminal card note](../../../.agents/notes/implemented/feature/2026-07-28-web-terminal-card.md).
 
+## Diff rendering
+
+`DiffBlock` renders a file mutation as an inline diff surface: one bold path header per file, the removed lines (`- `, error token) above the added lines (`+ `, success token), a `⋯` gap before a same-file second hunk, and a dim `└ +A -R · N file(s)` footer. Lines are `white-space: pre` with horizontal scrolling, so a source line holds its indentation instead of soft-wrapping, and the body collapses to a head slice plus a tail slice past `maxLines` (default 16, `TerminalBlock`'s split arithmetic) behind an expand button. A create (`oldText: null`) has no removed side. The copy control writes the prefixed diff text (path headers, `- `/`+ ` lines, the gap) so a multi-file copy stays attributable, and floats in the top-right corner rather than on a banner row of its own. Geometry mirrors `CodeBlock`/`TerminalBlock`. The `+`/`-` block form mirrors the TUI transcript's diff card so a diff reads the same across front ends. Rationale: [the web diff card note](../../../.agents/notes/implemented/feature/2026-07-30-web-diff-card.md).
+
 ## Model Experience
 
 None, as the package renders pure React atoms in the browser; nothing here reaches a model request.