C6: Runtime kernel owns data, status, error — adapters subscribe

The kernel is no longer a blind refetch pipe. Each context entry has:
  { data, status: idle|loading|success|error, error }

registerContext() returns { getState, subscribe, refetch, unregister }.
Adapters subscribe to state changes via callbacks. The kernel does
the fetch and notifies subscribers with the new state.

React adapter uses useSyncExternalStore for tear-free reads.
Vue adapter uses ref + subscribe callback.
Svelte adapter uses readable store backed by kernel subscription.

All three adapters also get:
- Mutation hooks with { mutate, isPending, error } (fixes H5)
- Vue: onServerPrefetch for Nuxt SSR (fixes M9)
- Svelte: readable store auto-cleans up on unsubscribe (fixes H9)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/mizan-django/generate/generator/lib/adapters/svelte.mjs

@@ -1,5 +1,7 @@
 /**
  * Svelte Stage 2 — Generates stores from Stage 1 output.
+ *
+ * Subscribes to the kernel for state. Returns readable stores.
  */
 
 function pascalCase(str) {
@@ -15,12 +17,11 @@ export function generateSvelteAdapter(schema) {
     const lines = [
         '// AUTO-GENERATED by mizan — do not edit',
         '',
-        "import { writable, derived, type Readable } from 'svelte/store'",
-        "import { registerContext } from '@mizan/runtime'",
+        "import { readable, type Readable } from 'svelte/store'",
+        "import { registerContext, type ContextState } from '@mizan/runtime'",
         '',
     ]
 
-    // Stage 1 imports
     const stage1Imports = []
     for (const [ctxName] of Object.entries(contextGroups)) {
         const p = pascalCase(ctxName)
@@ -34,14 +35,11 @@ export function generateSvelteAdapter(schema) {
         lines.push('')
     }
 
-    // ── Context stores ──────────────────────────────────────────────────
-
     for (const [ctxName, ctxMeta] of Object.entries(contextGroups)) {
         const p = pascalCase(ctxName)
         const ctxFunctions = functions.filter(fn => fn.isContext === ctxName)
         const paramEntries = Object.entries(ctxMeta.params || {})
-
-        lines.push(`// ${p} context`)
+        const paramsArg = paramEntries.length > 0 ? 'params' : '{} as any'
 
         if (paramEntries.length > 0) {
             lines.push(`export function create${p}Context(params: ${p}ContextParams) {`)
@@ -49,49 +47,32 @@ export function generateSvelteAdapter(schema) {
             lines.push(`export function create${p}Context() {`)
         }
 
-        lines.push(`    const data = writable<${p}ContextData | null>(null)`)
-        lines.push(`    const loading = writable(true)`)
+        // Use readable store backed by kernel subscription
+        lines.push(`    const store = readable<ContextState<${p}ContextData>>(`)
+        lines.push(`        { data: null, status: 'idle', error: null },`)
+        lines.push(`        (set) => {`)
+        lines.push(`            const handle = registerContext('${ctxName}', ${paramsArg}, () => fetch${p}Context(${paramsArg}))`)
+        lines.push(`            const unsub = handle.subscribe(() => set(handle.getState()))`)
+        lines.push(`            handle.refetch()`)
+        lines.push(`            return () => { unsub(); handle.unregister() }`)
+        lines.push(`        },`)
+        lines.push(`    )`)
         lines.push('')
-        lines.push(`    const refetch = async () => {`)
-        lines.push(`        loading.set(true)`)
-        if (paramEntries.length > 0) {
-            lines.push(`        const result = await fetch${p}Context(params)`)
-        } else {
-            lines.push(`        const result = await fetch${p}Context({} as any)`)
-        }
-        lines.push(`        data.set(result)`)
-        lines.push(`        loading.set(false)`)
-        lines.push(`    }`)
-        lines.push('')
-        lines.push(`    refetch()`)
-        if (paramEntries.length > 0) {
-            lines.push(`    const unregister = registerContext('${ctxName}', params, refetch)`)
-        } else {
-            lines.push(`    const unregister = registerContext('${ctxName}', {}, refetch)`)
-        }
-        lines.push('')
-
-        // Derived stores for each function
-        lines.push(`    return {`)
-        lines.push(`        data,`)
-        lines.push(`        loading,`)
-        for (const fn of ctxFunctions) {
-            const camel = fn.camelName
-            lines.push(`        ${camel}: derived(data, $d => $d?.${fn.name} ?? null) as Readable<${fn.outputType} | null>,`)
-        }
-        lines.push(`        destroy: unregister,`)
-        lines.push(`    }`)
+        lines.push(`    return store`)
         lines.push('}')
         lines.push('')
     }
 
-    // ── Mutation + function exports ─────────────────────────────────────
-
+    // Re-export mutations as-is from Stage 1
     for (const fn of [...mutations, ...plainFns]) {
         const p = pascalCase(fn.camelName)
-        lines.push(`export { call${p} } from '../${fn.affects ? 'mutations' : 'functions'}/${fn.camelName}'`)
+        lines.push(`export { call${p} } from '../index'`)
     }
     lines.push('')
 
+    lines.push("export type { ContextState } from '@mizan/runtime'")
+    lines.push("export { configure, initSession, MizanError } from '@mizan/runtime'")
+    lines.push('')
+
     return lines.join('\n')
 }