Inert hydration data mode for hash-based CSP on cached SSR pages #4174

Split out of #4028 (proposal 1). Deferred until after 2.0.

Problem

Hydration data is currently executable JS: each flush writes an inline script that assigns seroval output into _$HY.r. Fragment swaps ($df), head ops, the shell-end marker and redirects are inline scripts too.

Under a strict CSP an inline script runs only with a per-request nonce or a listed hash. A cached or CDN-served SSR page can't carry a fresh nonce. Hashes don't help either, because the data scripts' bodies differ per page, so the CSP header can't be written ahead of time.

#4161 made async-mode output deterministic, so the same data now gives the same bytes and the same hashes. That doesn't help when the data itself differs.

Idea

• Inert data: emit values as type="application/json" script blocks. The browser never executes them, so CSP doesn't apply.
• Fixed pump: one runtime script whose bytes are identical on every render, so its hash is a build-time constant for a static CSP header. It decodes the JSON blocks into _$HY.r and settles streamed promises. Streaming chunks are each followed by the same constant trigger script.
• Today's executable tasks (fragment swaps, head ops, shell-end marker) become data records the pump applies. The redirect becomes a meta refresh.

Cost and constraints

• JSON can't express seroval's JS output (shared and cyclic refs, promises, Maps, Dates), so the data uses the JSON data-table format (createJSONSerializer / createJSONDataTable) plus a client decoder.
• This is a second hydration wire format to keep in sync on both server (packages/web/src/server.ts) and client (packages/solid/src/client/hydration.ts, packages/web/src/client.ts, the bootstrap runtime).
• It must be opt-in and pay-for-use: apps that don't enable it should ship zero extra bytes, with the decoder tree-shaken away.
• Rough estimate: 600–1,000 lines plus byte-exact fixtures.

Who needs it: apps serving cached or static SSR HTML under a hash-based CSP. Nonce-based CSP works today.