fix(client): ignore nested katex tables for line highlights

[cyphercodes]

Summary

• Fix KaTeX line highlighting so nested tables from matrices/substacks are not treated as top-level equation rows.
• Move the row-collection logic into a small helper and add regression coverage for nested KaTeX tables.

Fixes #2559

Test plan

• pnpm exec vitest run packages/client/builtin/KaTexBlockWrapper.test.ts packages/slidev/node/syntax/katex.test.ts
• pnpm exec eslint packages/client/builtin/KaTexBlockWrapper.vue packages/client/builtin/katex-lines.ts packages/client/builtin/KaTexBlockWrapper.test.ts
• pnpm exec tsc --noEmit --target ESNext --lib DOM,ESNext --module ESNext --moduleResolution bundler --strict --skipLibCheck --types vitest,node packages/client/builtin/katex-lines.ts packages/client/builtin/KaTexBlockWrapper.test.ts
• pnpm --filter @slidev/types build && pnpm --filter @slidev/parser build && pnpm --filter @slidev/cli build && pnpm -C demo/starter run build

Note: pnpm typecheck currently stops in docs/node_modules/vitepress with missing declaration-file diagnostics before reaching the touched files.

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commit: 7ff2e10