perf(docs): preload WASM module in background on site visit

[pratyush618]

Summary

• WASM module now loads in the background via requestIdleCallback as soon as the user visits any page on the docs site
• By the time they navigate to the playground, the module is already cached — no spinner, instant interaction
• Uses Docusaurus clientModules to trigger the preload, and a shared cache consumed by useReclink

Test plan

• Visit any docs page, check Network tab — WASM files fetched during idle time
• Navigate to playground — should load instantly with no spinner
• Hard refresh on a playground page — still works (falls back to on-demand loading)