SSR + strict CSP for cached pages: let hydration data be inert so inline-script hashes are build-time constants (and streaming works under a hash-based CSP) #4028

Problem. We run a Solid 2 storefront (rc.13, @solidjs/vite-plugin start mode) behind a
shared CDN cache, with a CSP that has no 'unsafe-inline'. Three facts collide:

1. A nonce is unusable on a cached page. The stored response publishes its CSP header and its
   <script nonce> attributes together, to every later visitor, for the whole cache lifetime. The
   published constant becomes the grant. SvelteKit forbids nonces on prerendered pages for the
   same reason, and Next.js documents that nonces force dynamic rendering and disable static, ISR
   and CDN caching.
2. Hashes are the remaining mechanism, but Solid's inline scripts cannot be hashed ahead of
   time.
   • Every script that carries hydration data is per render: the seroval cross-reference header,
     the stub batch, the resolutions, $df("k"), head ops, window.location=.
   • The constant runtimes (REPLACE_SCRIPT, HEAD_SCRIPT, seroval's promise-resolver factory)
     are concatenated into the same element as that data.
   • Only HydrationScript is a standalone constant.
     So a host must render, re-read the finished body, and hash the scripts. That only works with
     the async render mode, where the head is sent after the body.
3. Streaming is impossible under a hash-based CSP. The header is gone before the late scripts
   exist.

A side effect we measured: in async mode, the order of the resolution chunks (and their $R ids)
follows the order in which async sources settle. Identical data with different API latency gives
different bytes (12 renders, 2 variants), so a body-digest ETag misses revalidations that should
be 304s.

Proposals, ranked.

1. Inert hydration data plus constant executable code (opt-in, for example
   renderToStream(…, { dataMode: 'json' })):
   • Write every serialized record as <script type="application/json" data-hy> using the JSON
     codec @solidjs/web/serialization already ships (createJSONSerializer /
     createJSONDataTable).
   • Keep all executable bytes constant per build: the runtime in its own <script>, or an
     external module, and a fixed pump such as <script>_$HY.p()</script> after each flush.
   • Make the late redirect a <meta http-equiv="refresh"> or a data record.
   • Two details to handle: the decoder must stamp p.s / p.v on settled promises, as
     PROMISE_SUCCESS does today, since hydration reads them synchronously; and seroval's
     cross-JSON already resolves out-of-order promise nodes by id.
     Result: CSP hashes are build-time constants (a static header, no body re-parse), and streaming
     SSR works under a strict, hash-only CSP. The cost from our measurement is about 5% larger
     payloads after compression and slower decode (0.7 vs 0.1 ms for a 120-product payload on a
     laptop).
2. Interim, small: the renderer records the SHA-256 of every inline script it writes and
   exposes the set on the settled result (or the request event), for hosts that render in async
   mode or with renderToString. Authored <script> in JSX stays the app's job, since the
   compiler bakes it into the template.
3. Determinism: in async mode, emit resolutions in a stable order (for example, by
   serialization key at flush) rather than in settle order, so identical data gives identical
   bytes.

Prior art.

• SvelteKit: csp.mode: 'hash' | 'nonce' | 'auto'. The framework adds hashes or nonces for
  the inline scripts and styles it generates. auto uses hashes for prerendered pages and nonces
  for dynamic ones, and "using nonces with prerendered pages is insecure and therefore forbidden".
• Astro 6: security.csp computes SHA-256 hashes of the page's scripts and styles and emits
  them in a <meta http-equiv> policy.
• Next.js: nonces only for inline scripts, which require dynamic rendering. Experimental SRI
  covers external files only.
• React Router / Remix and Qwik: nonce props only.
• SolidStart v2 already moved server-function payloads to seroval's JSON mode by default
  (solid-start#2042, after Is there any documentation about using the path syntax for stores? #1825).

What we would contribute.

• A failing test for each claim above: the per-render inventory, and the settle-order variance.
• A prototype of (1) behind an option: the serializer switch, the pump and the decoder wrapper
  that stamps s / v.
• Size and decode benchmarks on a real catalogue page.
• The docs section on running Solid behind a shared cache with a strict CSP.

We are happy to start with (2) or (3) if (1) is too large for 2.0.