fix(acp): relativize the completed diff card title

The result-time diff card sent view.title raw, so a completed edit/write
of an absolute in-workspace path flipped the card header back from the
relativized `Edit src/b.ts` to the absolute path — the pending card
relativizes, the result did not, and tool_call_update.title replaces the
header. Apply displayTitle to the result diff arm using the diff path,
mirroring the call-side card. Regression test proven red on the unfixed
arm.

Also record the overwrite diff-basis pre-read as a bounded follow-up
(TODO(overwrite-diff-bound) + RFC non-goal): overwriting a large file
reads the whole prior text into memory for a UI-only diff.

docs/rfc/implemented/architecture/2026-07-02-result-time-applied-hunk-diffs.md

@@ -46,6 +46,7 @@ Computing hunks-with-context is a solved problem with sharp edge cases (grouping
 - **Live incremental diff streaming.** The hunk is computed once, after the mutation completes; there is no per-keystroke diff.
 - **Diffing a binary/non-UTF-8 overwrite.** `before` is `null` for such a file (it has no text diff basis); the write still succeeds and the result renders a whole-file diff (`oldText: null`) rather than a contextual hunk.
 - **Rename/move diffs.** Only content diffs of a single resolved path.
+- **Bounding the overwrite diff basis.** An overwrite reads the whole prior file into memory to compute the contextual hunk (on top of the new content already held), so a very large text overwrite allocates both texts for a UI-only diff. A future refinement can bound the pre-read and fall back to a whole-file / no contextual diff above a size threshold; tracked as `TODO(overwrite-diff-bound)` at the read site.
 
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