feat!: migrate adapter to SvelteKit 3 (beta)

[ivanbarlog]

Summary

Migrates the adapter to SvelteKit 3, targeting v3 only (drops SvelteKit 2 support). Ships as a beta for testing against expense-ease.

Changesets beta pre-mode is active (.changeset/pre.json), so the Version Packages PR will compute a -beta.N version. With the pending minor (SK3 migration) + patch (keep-active) changesets, this resolves to 0.9.0-beta.0 under the beta dist-tag.

Adapter changes

• Replace the removed builder.generateManifest + "SERVER"/"MANIFEST" string-substitution flow with builder.generateServerInstance(...). The emitted module exports a ready server object, so the node/bun handler & stream entries now import { server } from "SERVER" and call server.init(...) / server.respond(...) instead of constructing the deprecated Server class.
• Read paths.base from the flattened builder.config.paths.base (builder.config.kit was removed).
• Replace deprecated builder.mkdirp / builder.rimraf with node:fs.
• Rework server instrumentation to the new flow: createInstrumentationInitializer(...) → include initializer in the bundle → per-entrypoint builder.instrument(...).
• Bump @sveltejs/kit peer dependency to ^3.0.0; pin dev dep to 3.0.0-next.31.

Examples

• Move config out of svelte.config.js (unsupported in v3) into the sveltekit({...}) Vite plugin; delete the svelte.config.js files.
• Replace deprecated json() with Response.json(...).

Verification

• bun run check (oxfmt + oxlint): 0 warnings, 0 errors
• bun test: 75 pass, 0 fail (47 AWS-gated integration tests skipped)
• bun --bun run build.ts: succeeds
• tsc --noEmit: passes against the real v3 types

Follow-ups before/after beta

• Peer-dep warning: ^3.0.0 does not match -next prereleases under semver, so consumers on a v3 prerelease see a benign peer warning during install.
• Instrumentation is untested end-to-end (expense-ease uses no instrumentation.server.*); validate on a real deploy if a consumer enables it.
• Sourcemap/stack-trace mapping in CloudWatch should be validated once expense-ease runs on the beta (build-time re-bundle with sourcemap: "linked").
• Exit beta pre-mode (changeset pre exit) before cutting the stable 0.9.0.