perf: cache startup filesystem scans, optimize base64 and path normalization

[Divkix]

Problem

Several hot paths in vinext perform redundant work:

1. hasMdxFiles() recursively walks app/ and pages/ via fs.readdirSync() on every config() hook invocation (fires 3+ times per Vite environment: RSC/SSR/Client)
2. resolvePostcssStringPlugins() probes 17 PostCSS config file candidates with fs.existsSync() on every config() hook invocation, with no caching
3. KV cache handler uses byte-by-byte String.fromCharCode/charCodeAt loops for base64 encode/decode instead of Buffer APIs
4. App Router prod server normalizes the request pathname, then discards it when constructing the Web Request — forcing the RSC handler to re-normalize

Solution

• Cache hasMdxFiles() and resolvePostcssStringPlugins() results per root directory
• Replace manual base64 with Buffer.from().toString("base64") / Buffer.from(str, "base64")
• Pass pre-normalized URL to nodeToWebRequest() so the RSC handler hits the fast path

PR

#436