perf(admin): take the meta schema out of the isolate (−41 MB warm)

[igoramf]

The admin JSON Schema costs ~40 MB of a 128 MB Workers isolate on a large site (montecarlo: 1690 blocks), and only GET /live/_meta reads it — nothing on the render path does. V8 keeps the module's source text and materialises the JSON.parse argument as a second independent string, both two-byte because the content is accented, so a 10.6 MB chunk is paid roughly four times over.

This mirrors what fastDeploy already does for the decofile: meta:<id> + meta:etag:<id> seeded by deco-sync-blocks-to-kv, decoVitePlugin({ metaFromKV: true }) stubbing meta.gen out of the server bundle, and handleMeta streaming kv.get(..., { type: "stream" }) straight into the Response — nothing parses the payload, since materialising it even as a string would reintroduce most of the cost. The ETag is precomputed at build under its own small key, both so If-None-Match (what admin actually polls with) costs one tiny read and because hashing JSON.stringify(schema) is impossible once the schema has left the isolate; the same value is merged into the payload's etag field so the existing {...schema, etag} wire format survives a byte-for-byte passthrough.

Measured on montecarlo, production build in workerd, warm over 8 real routes: 70.5 MB → 29.5 MB. (The first commit, making the eager options.meta() load lazy, measured at 0 MB on its own — the source text and the string literal are both materialised at module load, before evaluation. It is kept because it makes handleMeta async, which is what lets it await KV, and because it removes a floating promise that made /live/_meta nondeterministic.)

Degrades safely, unlike the decofile stub: with no keys handleMeta falls back to the bundled schema; with the bundle stubbed and no keys it answers 503 and only the admin loses the schema — the site keeps serving. The read side is registered from createDecoWorkerEntry rather than a setup call sites must remember, since montecarlo never calls setupTanstackFastDeploy and a getter nobody calls is the same bug as no getter. next is unaffected: it never registers a KV getter, so it keeps the in-bundle path.

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