fix(a11y): make trust scores keyboard accessible

[XonkelX]

Summary

• expose trust and integrity values as named ARIA meters with descriptive value text
• make score indicators keyboard reachable with a visible focus treatment
• convert verification coverage to a semantic table and label the workspace filters
• document the keyboard and axe-core audit results

Why

The workspace presented confidence values visually, but the score context and ranges were not available programmatically and the indicators could not receive keyboard focus. The default workspace also contained two unlabeled filters that caused the page's critical axe violations.

Closes #64

Validation

• npm run lint — passes with 0 errors (14 pre-existing warnings)
• npm run build — passes
• axe-core 4.10.3, WCAG 2 A/AA and WCAG 2.1 A/AA:
  • default workspace: 26 passed checks, 0 critical violations
  • selected-company integrity view: 21 passed checks, 0 critical violations
• keyboard audit confirmed all trust and confidence meters are reachable in DOM order and show a 2px focus outline

Follow-up

The broader workspace still has pre-existing serious findings for low-contrast secondary text and some non-focusable scroll regions. Those are documented in finverify-terminal/frontend/ACCESSIBILITY.md and kept outside this focused change.

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