fix TUI diff context line accounting

.agents/notes/implemented/bug-fix/2026-07-31-tui-diff-context-line-accounting.i18n.yaml

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+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# side as of the last confirmed-consistent state. Both languages carry equal authority;
+# after editing either side, bring the other along and re-record with:
+#   pnpm run verify-translation-pairing --write .agents/notes/implemented/bug-fix/2026-07-31-tui-diff-context-line-accounting.md
+2026-07-31-tui-diff-context-line-accounting.md: 71593022b56d9e675025f3a7a6d1e5e3edfa9b57
+2026-07-31-tui-diff-context-line-accounting.zh.md: df374c23b73fc5667cf733b4916d9d0d2ecb9198

.agents/notes/implemented/bug-fix/2026-07-31-tui-diff-context-line-accounting.md

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+# Agent Note: TUI diff context lines stay neutral
+
+Status: implemented
+
+English | [中文](2026-07-31-tui-diff-context-line-accounting.zh.md)
+
+## Problem
+
+Result-time filesystem diffs carry the applied change with three surrounding context lines in each `FileDiff.oldText` and `FileDiff.newText`. The TUI rendered every old-side row as removed and every new-side row as added, including the identical context present on both sides. A one-line edit therefore appeared as seven removals plus seven additions, and the footer repeated those inflated totals.
+
+## Decision
+
+The TUI compares each non-create `FileDiff.oldText` and `FileDiff.newText` at render time. Added and removed rows retain their green `+` and red `-` markers; equal context rows use the recessed body tone with a neutral two-space prefix. The footer sums only the rows classified as added or removed. A create (`oldText: null`) continues to classify every non-empty new-content row as added.
+
+This remains a consumer-side interpretation of the existing `FileDiff` contract. Filesystem tools continue to persist contextual before/after snippets, so other consumers keep their placement context and existing session logs replay with corrected TUI presentation. The TUI uses the same maintained `diff` package as `dsh-tool-fs` instead of introducing a second line-diff implementation.
+
+## Alternatives considered
+
+**Remove context from filesystem result metadata.** Rejected: contextual applied hunks are intentional producer output used by capable editors, and changing them would weaken every consumer while leaving old session logs misleading in the TUI.
+
+**Extend `FileDiff` with persisted per-line tags.** Rejected: the tags can be derived deterministically from the existing before/after pair; persisting them would widen the cross-package and session-log contract solely for one renderer.
+
+**Match equal lines by position without a diff algorithm.** Rejected: insertions and deletions shift subsequent context, so positional pairing would misclassify valid hunks.
+
+## Consequences
+
+TUI diff cards distinguish evidence-bearing context from the mutation itself, and their `+A -R` footer reports the actual line delta. Replaying an existing contextual diff gains the corrected rendering without a migration. Rendering performs one additional line comparison per non-create hunk; result-time hunks are already context-bounded, while create cards bypass the comparison.
+
+The focused TUI test covers neutral context and exact totals. The assembled `advanced-cards` terminal snapshots pin the neutral context style, semantic change colors, and `+1 -1` footer through collapsed and expanded card states.

.agents/notes/implemented/bug-fix/2026-07-31-tui-diff-context-line-accounting.zh.md

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+# Agent Note: TUI diff 上下文行保持中性
+
+Status: implemented
+
+[English](2026-07-31-tui-diff-context-line-accounting.md) | 中文
+
+## 问题
+
+文件系统 diff 返回结果时，每个 `FileDiff.oldText` 和 `FileDiff.newText` 都会包含已应用的变更及其前后各 3 行上下文。TUI 将旧侧的每一行都渲染为删除行，将新侧的每一行都渲染为新增行，其中包括两侧相同的上下文。因此，一行编辑会显示为删除 7 行并新增 7 行，页脚还会重复这些虚高的合计值。
+
+## 决策
+
+对于每个不对应文件创建的 `FileDiff`，TUI 在渲染时比较 `FileDiff.oldText` 和 `FileDiff.newText`。新增行和删除行仍分别使用绿色 `+` 和红色 `-` 标记；相同的上下文行则使用弱化的正文色调，并带有由两个空格构成的中性前缀。页脚只汇总归类为新增或删除的行。创建操作（`oldText: null`）仍将新内容中的每个非空行归类为新增行。
+
+该行为仍然只是消费方对现有 `FileDiff` 契约的解释。文件系统工具仍会持久化带上下文的变更前后片段，因此其他消费方仍能获得定位上下文，已有会话日志在回放时也会采用修正后的 TUI 呈现。TUI 与 `dsh-tool-fs` 共用同一个受维护的 `diff` 包（package），无需引入第二套逐行 diff 实现。
+
+## 考虑过的替代方案
+
+**从文件系统结果元数据中移除上下文。** 不予采纳：带上下文的已应用 hunk 是有意保留的生产方输出，供具备相应能力的编辑器使用；更改这些内容会让所有消费方丢失信息，同时旧会话日志在 TUI 中仍会产生误导。
+
+**为 `FileDiff` 扩展持久化的逐行标签。** 不予采纳：这些标签可以根据现有的变更前后文本对确定性派生；仅为一个渲染器持久化标签，会扩大跨包契约和会话日志契约。
+
+**不使用 diff 算法，按位置匹配相同行。** 不予采纳：插入和删除会使后续上下文发生位移，因此按位置配对会把有效 hunk 错误分类。
+
+## 后果
+
+TUI diff 卡片会区分用于佐证的上下文与变更本身，其 `+A -R` 页脚报告实际的行变更量。回放已有的上下文 diff 无需迁移即可获得修正后的渲染。渲染每个不对应文件创建的 hunk 时，会额外执行一次逐行比较；结果时刻的 hunk 本就受上下文范围限制，创建卡片则会跳过比较。
+
+聚焦的 TUI 测试覆盖中性上下文和精确合计值。组装后的 `advanced-cards` 终端快照在卡片折叠和展开状态下固定了中性上下文样式、变更行的语义色彩，以及 `+1 -1` 页脚。