feat(inspector): seek the Timeline from a governor usage chart

[lukecotter]

📝 PR Overview

The inspector's governor usage charts show when a metric climbed, but the chart was a picture only. A spike at 4s gave no way to reach the code behind it: you read the time off the chart, then hunted for it on the Timeline by hand.

The charts are now seekable. Click a point, or step a focused chart with the arrow keys and press Enter, and the Timeline moves to that instant and zooms to a window of max(2% of the log, 100ms) around it — 480ms on the 24s sample log. Nothing is selected, so the inspector keeps the whole-log reading the charts belong to.

🛠️ Changes made

• GovernorTrends emits timeline:navigate-to with a new seek mode. A click takes the pointer position; the arrow keys step one seek window at a time, so no part of the log falls between reachable windows.
• New seekWindow(at, logTotal) owns the rule, so the width comes from the log and never from whatever frame the instant lands in.
• ApexLogTimeline.navigateToTimestamp gains the seek branch: zoom with no selection, and no padding, so the window shown is the width asked for. A seek with no frame spanning the instant still moves, centred at depth 0.
• timeline:navigate-to narrows so mode sits only on the timestamp form: only an instant can be sought.
• Each chart is a <button>, so the focus ring shows for keyboard focus and not for a click, and the readout is a live region, so a stepped cursor is announced.

🧩 Type of change (check all applicable)

• 🐛 Bug fix - something not working as expected
• ✨ New feature – adds new functionality
• ♻️ Refactor - internal changes with no user impact
• ⚡ Performance Improvement
• 📝 Documentation - README or documentation site changes
• 🔧 Chore - dev tooling, CI, config
• 💥 Breaking change

📷 Screenshots / gifs / video [optional]

🔗 Related Issues

related #373

✅ Tests added?

• 👍 yes
• 🙅 no, not needed
• 🙋 no, I need help

📚 Docs updated?

• 🔖 README.md
• 🔖 CHANGELOG.md
• 📖 help site
• 🧪 Marked any pre-release-only features (README 🧪 badge — see RELEASING.md)
• 🙅 not needed

Anything else we need to know? [optional]

Test plan: pnpm lint, pnpm test, pnpm build, then sample-app/debug-logs/sample-log.log, Timeline tab, inspector with nothing selected:

1. Click the middle of a usage chart: the Timeline zooms to about 480ms around that instant, and nothing is selected.
2. Move the pointer along a chart: the readout follows the line and the Timeline does not move.
3. Tab to a chart, press ArrowRight a few times, then Enter: the Timeline moves to the cursor, not the last sample.
4. Move the mouse across the chart and away after step 3: the arrow-key cursor stays.
5. Click a chart: no focus ring.

Repeat in a light and a dark theme, and with the panel docked to the side and to the bottom.