web: turn file-change cards show no stats/diff for Write calls and replace_all Edits

Summary

In the web UI, the per-turn "N files changed" summary card aggregates file changes per tool call. Two cases currently produce no usable result:

1. Write calls: the file row shows no +x/-y stats, and clicking the row/card opens the raw file instead of a diff view.
2. Edit calls with replace_all: true: same symptom — no stats, no diff.

Additionally, when any file in the card is affected, the card header hides the aggregate +x/-y totals as well.

Steps to reproduce

1. Open the web UI against a running backend.
2. Run a session where the agent creates a file with the Write tool (or performs an Edit with replace_all: true).
3. Look at the turn's file-change card: the affected file row has no stats; clicking it opens the file, not a diff.

Root cause (from the shipped web bundle, 0.37.2)

Inspecting the bundled frontend code, the turn-changes aggregation works roughly like:

• The aggregator routes write to a dedicated branch that sets hasWrite = true and statsIncomplete = true and never computes a diff (only edit/multi_edit go through the diff builder).
• The diff-builder helper returns null for anything that is not edit/multi_edit, and explicitly returns null for edits with replace_all === true.
• The card's click handler decides openFile vs openDiff based on hasWrite, rather than on whether a computed diff actually exists.
• The header aggregate is hidden when any entry has statsIncomplete.

So the data (path, content / old_string / new_string) is all present in the history frames — this is purely a presentation-layer gap.

Suggested behavior

• Write (new file / overwrite): build an all-additions diff from "" -> content, matching what #2274 proposed for the live preview panel. Show stats from that diff.
• Edit with replace_all: true: build a representative diff from old_string -> new_string (a single-occurrence view is still far more useful than nothing).
• Click handler: prefer openDiff whenever a diff exists, regardless of hasWrite.
• Longer term, the ideal fix is server-side: record the real before/after content at execution time so overwrite Writes, append mode, and replace_all multiplicities can be rendered accurately instead of approximated from args.

Environment

• kimi-code 0.37.2 (single-binary), macOS arm64
• Web UI in Chrome

Related

• #2268 / #2274 covered the Write live preview panel (closed unmerged); this issue is about the turn summary card, which is a separate code path.

Happy to provide bundle-level details (function shapes / exact guards) if useful.

Start by locating the turn-change aggregator, diff-builder, and card click handler in the web frontend, then reproduce the Write and replace_all cases against a running backend. Trace the existing edit/multi_edit diff path and the related live preview behavior. Done means affected rows show stats, open diffs, and the card header retains aggregate totals when a diff can be computed.