Closes #147

Summary

• Adds a primary diff verdict that explains the likely driver: duration, cost, failures, tokens/context, tool count, model change, or health drop.
• Adds direction-aware delta chips for cost, tokens, duration, turns, tools, failures, and health.
• Keeps the side-by-side diff panels below the explanation so raw metrics remain available.

User value

Users no longer have to infer why one run is slower, costlier, or lower quality from raw fields; the diff view now states the likely reason first and backs it with deltas.

Scope

• TUI diff view narrative and delta presentation.
• Adds token count as a diff field for context-pressure comparison.
• English/Chinese labels for verdicts and token deltas.

Changed files

• internal/engine/engine.go
• internal/tui/insights.go
• internal/tui/tui.go
• internal/tui/tui_test.go
• internal/i18n/i18n.go

Test plan

• go test ./internal/tui ./internal/engine
• go test ./...
• go build -o /tmp/agenttrace-parser-check ./cmd/agenttrace
• /tmp/agenttrace-parser-check --doctor || true
• /tmp/agenttrace-parser-check --demo --overview -f json
• go test ./internal/tui -run 'TestDiffRendersWinnerInsight|TestWideDiffUsesFullComparisonLayout' -count=1 -v

Fixture/privacy note

No real user session data, prompts, logs, tokens, or secrets were added. Tests use existing synthetic in-memory TUI fixtures. Local doctor validation saw 4264 session files, but no local session content was committed.

Risk

Medium-low. This adds a token diff field and changes TUI diff ordering/copy. Parser/source compatibility, CLI behavior, and JSON/Markdown/HTML report contracts are unchanged.

Non-goals

• No parser/source changes.
• No report contract changes.
• No README/docs asset changes.
• No release, package, install, or launch-kit changes.

Blackboard events

• Claimed [TUI] Make diff view explain why one run is slower or costlier #147 with: Parser & Implementation Agent is taking this.
• Created and pushed branch parser/147-diff-explains-change.
• Implemented primary diff verdict and direction-aware key delta chips.
• Terminal capture evidence: focused diff tests passed with -v.

Release impact

patch; affects TUI visible behavior only. Parser/source compatibility: no. CLI behavior: no. JSON/Markdown/HTML report contract: no. Install/package docs: no.

Handoff suggestion: Growth

Suggested release note: Diff view now explains why one run is slower, costlier, or lower quality, with direction-aware deltas for the key fields.