Investigate @babel/standalone for ES2020+ syntax repair in Playground (Chakra runtime)

## Background

Babylon Native's Chakra-based runtime parses only up to ES2019 syntax. Modern Babylon.js Playground snippets (sourced from the live Babylon playground server at runtime) routinely use:

- Optional chaining (`?.`)
- Nullish coalescing (`??`)
- Numeric separators (`1_000_000`)
- Logical assignment (`||=` `&&=` `??=`)
- Private class fields (`#field`)

These snippets fail with `SyntaxError` at the `eval()` site in `Apps/Playground/Scripts/validation_native.js`, causing ~26 visual tests to be excluded from automatic testing on Chakra Playground builds.

## Why this is hard

The transpilation has to run **inside** the JS host (Chakra) at parse-failure time, not at build time, because the snippet source is fetched dynamically from the Babylon playground server. So traditional Node.js-based tools (babel CLI, swc CLI) aren't directly usable.

A previous attempt (PR #1709, since closed) shipped a ~114-line regex-based syntax repair polyfill. It addressed the parse failures but introduced semantic divergence (`?.` rewritten to `.` loses null-short-circuit), and only unlocked **1 of 26** affected tests. The other 25 either hit runtime `TypeError`s from the divergence (9), need ES2022 features the regex doesn't cover (4), or hit non-transpilation blockers like missing modules (12).

## Options to investigate

| | Approach | Tradeoff |
|---|---|---|
| A | Bundle `@babel/standalone` into Playground SCRIPTS, run inside Chakra | ~1 MB extra JS shipped + parsed at startup; semantically correct; well-maintained |
| B | swc compiled to WebAssembly, run inside Chakra | Chakra does not support WASM in the BN-shipped version — likely not viable |
| C | Native binding to swc (Rust) via C++, expose to JS host | Adds native dependency + cross-platform binary build; significant infra |
| D | Build-time pre-transpile of cached snippets | Requires intercepting/caching Babylon CDN fetches; significant infra |

## Suggested deliverables for this investigation

1. Measure the actual cost of bundling `@babel/standalone` (size on disk, Chakra parse time at startup, memory footprint).
2. Prototype option A and confirm the ~14-test uplift estimate (the 9 `?.` divergence + 4 ES2022 + some asset/url tests that may benefit from cleaner transformation).
3. Compare against options C / D for long-term sustainability.
4. Decision + RFC.

## Out of scope

This issue is **only** about ES2020+ syntax repair in the Chakra Playground. The non-transpilation blockers (missing `babylonjs-addons`, missing `earcut`, asset/url errors) are tracked separately.