feat: add path params support to HTML loader API

[field123]

Summary

• Add support for dynamic route parameters ($ctx.params, $ctx.query) in the HTML Render API
• Enables SSR of dynamically-routed pages via the HTML loader

New Query Parameters

Parameter	Type	Description	Example
pageRoute	string	Route pattern with dynamic segments	/products/[slug]
pageParams	JSON	Path parameter values	{"slug":"my-product"}
pageQuery	JSON	Query string parameters	{"utm_source":"google"}

Changes

• Add page-params.ts module with pure validation/parsing functions
• Update gen-html-bundle.ts to accept and pass params to render functions
• Update loader.ts route handlers to parse new query params
• Update hydrater.ts to read data attributes and pass to hydrate

Known Limitations

• $ctx.pageRoute and $ctx.pagePath won't work after hydration due to limitations in the @plasmicapp/loader-react npm package
• $ctx.params and $ctx.query will work correctly

Test plan

• Test SSR with pageParams renders $ctx.params.slug correctly
• Test hydration preserves params
• Test backward compatibility (requests without new params still work)
• Test error handling for invalid JSON