feat: add trend indicators to PR comments and CI summary

[maximn]

Summary

Adds at-a-glance trend indicators to PR comments and CI summaries using cached history. When 3+ runs exist, trend arrows (↑ → ↓) and deltas appear for pass rate, duration, and test count. When 5+ runs exist, ASCII sparklines are also shown.

Changes

• src/history/types.ts — added TrendDirection type and TrendIndicators interface
• src/history/trends.ts (new) — computeTrends() function + shared buildSparkline() helper
• src/history/perf-regression.ts — refactored buildDurationSparkline() to use shared buildSparkline()
• src/output/pr-comment.ts — added renderTrendLine(), wired into renderTestJobSection() below stats line
• src/output/summary.ts — added renderTrendsSection(), inserted after metrics table in generateSummary()
• src/index.ts — compute trends when ≥3 entries, non-blocking try/catch, pass to summary + PR comment
• Tests — 41 new tests across unit, rendering, and integration layers (553 total, zero regressions)

PR comment format:

📈 Pass rate: 97.5% ↑ (+2.3%) · Duration: 12.4s ↓ (-1.2s, -8.8%) · Tests: 100 (+3)

CI summary format (with sparklines):

### Trends
**Pass rate:** ▃▅▅▆▇ 97.5% ↑ (+2.3%)
**Duration:** ▅▃▃▂▁ 12.4s ↓ (-1.2s)
**Tests:** 100 (+3)

Test plan

• Verify PR comment shows trend line when history has ≥3 runs
• Verify CI summary shows Trends section with arrows and deltas
• Verify sparklines appear only when ≥5 runs
• Verify trends are omitted entirely when history: false
• Verify trends are omitted when fewer than 3 runs exist
• Confirm action does not fail when trend computation throws

[github-actions]

E2E Results: https://github.com/testglance/action/actions/runs/23988650455