feat(detection): add alternatePackageNames so React Router v6 is detected

[Ref34t]

Why

React Router v6 ships the router as react-router-dom; v7 consolidated onto react-router. The detection gate only checked the v7 name:

return packageJson ? hasDeclaredDependency('react-router', packageJson) : false;

hasDeclaredDependency is an exact key lookup, so a v6 project — which declares only react-router-dom — never matched, and first-match detection carried it through to the javascript_web fallback.

The rest of the config already assumes v6 is supported: minimumVersion is '6.0.0', and getReactRouterMode in ./utils checks both package names and has an explicit V6 branch. Neither is reachable, because detection fails first. context-mill's dedicated react-react-router-6 skill variant is unreachable for the same reason, so a v6 project gets the generic javascript_web rules — whose pageview guidance is written for "SPAs without a framework router".

What this adds

An optional alternatePackageNames on FrameworkDetection, rather than special-casing React Router in the runner — following the guidance in .claude/skills/wizard-development: "the interface is missing a field — extend the interface, don't add special-case logic to the runner."

React Router declares ['react-router-dom'] there, and both names are now used in detect, getVersion, getInstalledVersion, and the is-it-installed warning in posthog-integration.

Test plan

• Two new cases in src/lib/detection/__tests__/framework.test.ts: a v6 project (react-router-dom only) and a v7 project both resolve to Integration.reactRouter.
• pnpm build && vitest run — 1558 tests pass across 113 files.
• tsc --noEmit — 27 pre-existing errors before and after, none in the touched files.
• Ran every framework's detect() predicate over each app in context-mill/example-apps: react-react-router-6 resolved to javascript_web before and react-router after. react-react-router-7-data, -declarative and -framework are unchanged, and no other app's result moved.