feat(tui): add service-scoped token usage graph

[edwin-zvs]

Summary

• show the project-style token usage graph in service views
• scope cost and compute-time samples to routed sessions and their descendants
• use one shared graph and legend renderer across operator, project, and service views
• preserve colored model dots and names, equal-width legend columns, per-model rates, overflow counts, summed rate, cache shading, and hover detail
• omit the service graph on short panes and exclude unrelated services
• document the shared legend behavior

Tests

• cargo test -p construct-cli service_view
• cargo test -p construct-cli project_dashboard
• cargo test -p construct-cli legend_
• cargo build
• git diff --check

Binary

/Users/moon/agentd/.claude/worktrees/service-view-token-usage/target/debug/construct