Fix H3, H6, H11, H13, M11, M18 — quick wins from expert review

H3: mizanFetch retries 2x on server errors (5xx) and network
failures. 200ms/400ms backoff. Mutations NOT retried (not idempotent).

H6: refreshContext now uses GET /ctx/<name>/ instead of POST /call/.
Context reads go to the context endpoint, not the mutation endpoint.

H11: Python cache key derivation normalizes True→"true",
False→"false", None→"null" for cross-language HMAC consistency
with JavaScript's String() behavior.

H13: Forms isValid now checks that all required fields have been
touched, not just that touched fields have no errors.

M11: execute_function return type updated to include HttpResponseBase
for view-path functions.

M18: registerContext cleanup uses ?. instead of ! to prevent crash
if Map was cleared (already fixed in H2 commit but documenting).

373 Django + 33 React tests pass.

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

packages/mizan-runtime/src/index.ts

@@ -151,6 +151,24 @@ function flush(): void {
 
 // === Fetch ===
 
+async function fetchWithRetry(
+    input: RequestInfo | URL,
+    init?: RequestInit,
+    retries = 2,
+): Promise<Response> {
+    for (let attempt = 0; ; attempt++) {
+        try {
+            const res = await fetch(input, init)
+            // Don't retry client errors (4xx) — only server/network errors
+            if (res.ok || (res.status >= 400 && res.status < 500)) return res
+            if (attempt >= retries) return res
+        } catch (e) {
+            if (attempt >= retries) throw e
+        }
+        await new Promise(r => setTimeout(r, (attempt + 1) * 200))
+    }
+}
+
 async function resolveHeaders(): Promise<Record<string, string>> {
     await initSession()
 
@@ -179,7 +197,7 @@ export async function mizanFetch(
     }
 
     const headers = await resolveHeaders()
-    const res = await fetch(url.toString(), { headers, credentials: 'same-origin' })
+    const res = await fetchWithRetry(url.toString(), { headers, credentials: 'same-origin' })
     if (!res.ok) throw new MizanError(res.status, await res.text())
     return res.json()
 }
@@ -191,6 +209,7 @@ export async function mizanCall(
     const headers = await resolveHeaders()
     headers['Content-Type'] = 'application/json'
 
+    // Mutations are not retried — they are not idempotent
     const res = await fetch(`${config.baseUrl}/call/`, {
         method: 'POST',
         headers,