Add TanStack Start landing page

[PerfectPan]

Summary

• Add a TanStack Start landing page under site/ with ocvm install flow, feature overview, docs links, and custom SVG mark.
• Add site-local Vercel configuration and documentation for preview deployment.
• Exclude site/** from the Rust crate package so publishing the CLI stays small.

Preview

• Vercel: https://site-1sk85z4yd-perfectpans-projects.vercel.app
• Alias: https://site-neon-two.vercel.app

Validation

• docker run --rm -u "$(id -u):$(id -g)" -e npm_config_cache=/tmp/npm-cache -v "$PWD":/work -w /work/site node:22-bookworm sh -lc "npm ci && npm run build"
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test
• cargo build --release
• cargo package --allow-dirty --offline -> 28 files, 29.1KiB compressed
• Playwright visual QA screenshots at 1440x1100 and 390x844
• publish-safety scan with generated lockfile/build artifact exclusions

Closes #10