fix: use :root font-size as compile-time rem base

[danstepanov]

Summary

• When no explicit inlineRem option is passed, the compiler now scans the CSS for :root { font-size: Npx } and uses that value as the rem multiplier instead of hardcoding 14
• This makes the documented v5 approach for overriding rem actually work without needing to pass inlineRem through metro config
• Explicit inlineRem option still takes precedence when set

Closes #316

Test plan

• New test: "rem - css root font-size override" verifies :root { font-size: 16px } produces 16 * 10 = 160 instead of default 14 * 10 = 140
• Existing "rem - default" test still passes (hardcoded 14 when no :root font-size)
• Existing "rem - inline override" test still passes (inlineRem option takes precedence)
• Existing "rem - css override" test still passes (inlineRem: false for runtime resolution)