fix: eagerly initialise dynamic env vars from process.env at module-eval time

[elliott-with-the-longest-name-on-github]

Previously, $app/env/private and $app/env/public dynamic variable values were only populated when Server.init() called set_env(). If any module reading these values was evaluated before Server.init() ran — for example via bundler chunk colocation with instrumentation.server.js (see #16288) — the values would be silently undefined.

What changed

builder.instrument() now accepts an env option. When provided, the generated facade creates a separate init module that imports set_env from the env module and calls it with the platform's env before instrumentation is imported. Since static imports are evaluated in order, this ensures env is populated (and validated) before any instrumentation or application code runs.

Each adapter passes the appropriate env source:

• adapter-node, adapter-vercel (serverless): process.env
• adapter-cloudflare: env from cloudflare:workers (available at module scope)
• adapter-vercel (edge), adapter-netlify (serverless): env init added to their existing generateText facades

If required env vars are missing, set_env will throw a clear validation error at startup — this is intentional, as the app cannot function without them.

Why?

This eliminates the entire class of "captured undefined" bugs regardless of when a module evaluates, and works across all platforms (not just Node-like ones) because each adapter provides its own env source.

Stacked on top of #16302 (which externalizes @opentelemetry/api to prevent the primary colocation vector). Both fixes are complementary:

• fix: externalize @opentelemetry/api to prevent chunk colocation #16302 fixes the isolation problem (prevents colocation via the OTEL shared dep).
• This PR fixes the value-timing problem (ensures env is populated even if colocation occurs via some other dep).

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 3508875:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/3508875d9d7fd77328f4c541ee97774f4d73e9bd

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16303

[changeset-bot]

🦋 Changeset detected

Latest commit: 3508875

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@sveltejs/kit	Patch
@sveltejs/adapter-node	Patch
@sveltejs/adapter-cloudflare	Patch
@sveltejs/adapter-vercel	Patch
@sveltejs/adapter-netlify	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16303-svelte.vercel.app/

[SobolievOleksii]

Tested this branch against the repro for #16288 (adapter-node + src/instrumentation.server.js): the server crashes at startup —

build/__sveltekit_env_init.js:2
import { set_env } from './env.js';
SyntaxError: The requested module './env.js' does not provide an export named 'set_env'

The facade written in builder.js hardcodes import { set_env } from './env.js' relative to the entrypoint, but for adapter-node build/env.js is the adapter's own runtime helper (export { env, env_prefix, timeout_env }) — and kit's env module with set_env doesn't end up at a stable relative path after the adapter's bundling step. Full details in #16288 (comment) — happy to re-test after an adjustment.

[elliott-with-the-longest-name-on-github]

Yeah, this is still a draft, not ready yet 😅