fix(web): diff card review — TUI parity, path-header overlap, double-resolve

Bring the TUI diff footer onto the same terminator rule and distinct-path
count the Web DiffBlock uses (a trailing newline terminates its line; two
hunks in one file read as 1 file), so the two front ends' `+A -R · N file(s)`
footers agree. Reserve space in the diff path header for the floating copy
button so a long path no longer scrolls under it. Pass the tool's raw path to
the injected openFile (which already resolves against cwd) instead of resolving
twice. Rename the shared block-body CSS class to a card-neutral cardBody so a
terminal-spacing tweak cannot silently move the diff card. Add a same-file
two-hunk TUI unit test and an assembled built-boot assertion that the write
turn renders +1 -0 · 1 file end to end.

.agents/notes/implemented/feature/2026-07-30-web-diff-card.md

@@ -16,11 +16,12 @@ This is the [terminal card](2026-07-28-web-terminal-card.md) done for the `diff`
 
 `DiffBlock` is a `ui-primitives` component that renders a file mutation as an inline diff surface, and both Web render sites for a write/edit call consume the diff render intent through it: the chat tool row's body and the details panel's Output section. `ui-conversation/src/client/contract/diff-card-model.ts` is the single place that turns the snapshot's `callView`/`resultView` pair into the component's props, so the two sites cannot disagree about a change. It returns null — the generic path — whenever neither side declares `card: 'diff'`, including a `card` value this client version does not know, and whenever a settled call's result view is generic, which is how write/edit keep their execution errors on the generic path. The result side is authoritative once the call settles: the applied hunks replace the call-time diff derived from the arguments alone. A paging window that drops the call head still renders, because the result view carries the whole change.
 
-The component's contract follows the TUI's `diffLines` (`packages/ui/tui/src/components/transcript.ts`) so a diff reads the same shape across front ends, with one deliberate divergence noted below (the file count):
+The component's contract follows the TUI's `diffLines` (`packages/ui/tui/src/components/transcript.ts`) so a diff reads the same shape across front ends:
 
-- **One path header per file.** A new file opens a bold path header; a same-file second hunk (a scattered edit, or a `replace_all`) opens with a `⋯` gap instead of repeating the path. The `N file(s)` footer counts DISTINCT paths — the divergence from the TUI, whose footer uses `diffs.length` and so reads two hunks in one file as `2 files` where this reads `1 file`.
+- **One path header per file.** A new file opens a bold path header; a same-file second hunk (a scattered edit, or a `replace_all`) opens with a `⋯` gap instead of repeating the path. The `N file(s)` footer counts DISTINCT paths on both front ends — this PR moved the TUI footer off `diffs.length` onto the distinct-path count, so two hunks in one file read as `1 file` in both.
 - **The change in the diff's own colors.** A removed line is `- ` on the error token, an added line is `+ ` on the success token, drawn verbatim with `white-space: pre` inside a horizontally scrolling box — a source line is read by its indentation, so it scrolls rather than folds. A create (`oldText: null`) has no removed side.
 - **Height cap with an expand control.** A diff longer than `DEFAULT_DIFF_MAX_LINES` (16) shows `ceil(max/2)` head rows plus the remaining tail rows, with a button between reporting the hidden count. The split arithmetic matches `TerminalBlock` and the TUI's collapsed card, so a long diff's head and tail slices agree across front ends.
+- **Line terminator.** A side's content splits on `\n` under the terminator rule `TerminalBlock` uses: empty text is zero lines (a full deletion's `newText`, a create's absent `oldText` side), a single trailing newline terminates its last line rather than adding a phantom empty one, and an interior blank line survives. This PR applied the same rule to the TUI diff branch, so the `+A -R` footer counts agree on both front ends for the newline-terminated content real write/edit calls carry.
 - **Footer and copy.** A dim `└ +A -R · N file(s)` footer summarizes the change; `+A -R` are the added/removed line counts, the same per-side counts the TUI footer draws. The copy control copies the prefixed diff text (path headers, `- `/`+ ` lines, the `⋯` gap), so a multi-file copy stays attributable.
 
 Geometry, radius, and fonts mirror `CodeBlock`/`TerminalBlock` so a diff card, a terminal card, and a fenced block read as one family; `white-space: pre` plus horizontal scroll is the deliberate divergence. The copy control floats in the card's top-right corner rather than on a banner row of its own, because a banner carrying only a copy button drew an empty band above the first diff line — the TUI diff card has no banner either, only the footer.