docs: correct SVA sequence method guidance

[thomasnormal]

Summary

• Correct the sva/triggered lesson's .triggered restriction using IEEE 1800-2023 §§16.9.11 and 16.13.6.
• Explain .matched as the cross-clock sequence method per §16.13.5.
• Add a failing-first Vitest guard for both claims.

Evidence

• src/lessons/sva/triggered/trigger_check.sol.sv: refdiff both_pass with native Mox and Xcelium.
• npm test: 85 passed, 4 skipped.
• npm run build: passed (existing Svelte/a11y and duplicate compressed-output warnings remain).

Browser-audit item: .triggered content restriction (lesson 57).